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ext4: fix races between page faults and hole punching

Currently, page faults and hole punching are completely unsynchronized.
This can result in page fault faulting in a page into a range that we
are punching after truncate_pagecache_range() has been called and thus
we can end up with a page mapped to disk blocks that will be shortly
freed. Filesystem corruption will shortly follow. Note that the same
race is avoided for truncate by checking page fault offset against
i_size but there isn't similar mechanism available for punching holes.

Fix the problem by creating new rw semaphore i_mmap_sem in inode and
grab it for writing over truncate, hole punching, and other functions
removing blocks from extent tree and for read over page faults. We
cannot easily use i_data_sem for this since that ranks below transaction
start and we need something ranking above it so that it can be held over
the whole truncate / hole punching operation. Also remove various
workarounds we had in the code to reduce race window when page fault
could have created pages with stale mapping information.

Signed-off-by: Jan Kara <jack@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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Jan Kara authored and tytso committed Dec 7, 2015
1 parent f41683a commit ea3d7209ca01da209cda6f0dea8be9cc4b7a933b
Showing with 127 additions and 42 deletions.
  1. +10 −0 fs/ext4/ext4.h
  2. +30 −24 fs/ext4/extents.c
  3. +57 −9 fs/ext4/file.c
  4. +27 −9 fs/ext4/inode.c
  5. +1 −0 fs/ext4/super.c
  6. +2 −0 fs/ext4/truncate.h
View
@@ -910,6 +910,15 @@ struct ext4_inode_info {
* by other means, so we have i_data_sem.
*/
struct rw_semaphore i_data_sem;
/*
* i_mmap_sem is for serializing page faults with truncate / punch hole
* operations. We have to make sure that new page cannot be faulted in
* a section of the inode that is being punched. We cannot easily use
* i_data_sem for this since we need protection for the whole punch
* operation and i_data_sem ranks below transaction start so we have
* to occasionally drop it.
*/
struct rw_semaphore i_mmap_sem;
struct inode vfs_inode;
struct jbd2_inode *jinode;
@@ -2484,6 +2493,7 @@ extern int ext4_chunk_trans_blocks(struct inode *, int nrblocks);
extern int ext4_zero_partial_blocks(handle_t *handle, struct inode *inode,
loff_t lstart, loff_t lend);
extern int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf);
extern int ext4_filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf);
extern qsize_t *ext4_get_reserved_space(struct inode *inode);
extern void ext4_da_update_reserve_space(struct inode *inode,
int used, int quota_claim);
View
@@ -4770,7 +4770,6 @@ static long ext4_zero_range(struct file *file, loff_t offset,
int partial_begin, partial_end;
loff_t start, end;
ext4_lblk_t lblk;
struct address_space *mapping = inode->i_mapping;
unsigned int blkbits = inode->i_blkbits;
trace_ext4_zero_range(inode, offset, len, mode);
@@ -4785,17 +4784,6 @@ static long ext4_zero_range(struct file *file, loff_t offset,
return ret;
}
/*
* Write out all dirty pages to avoid race conditions
* Then release them.
*/
if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
ret = filemap_write_and_wait_range(mapping, offset,
offset + len - 1);
if (ret)
return ret;
}
/*
* Round up offset. This is not fallocate, we neet to zero out
* blocks, so convert interior block aligned part of the range to
@@ -4856,16 +4844,22 @@ static long ext4_zero_range(struct file *file, loff_t offset,
flags |= (EXT4_GET_BLOCKS_CONVERT_UNWRITTEN |
EXT4_EX_NOCACHE);
/* Now release the pages and zero block aligned part of pages*/
truncate_pagecache_range(inode, start, end - 1);
inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
/* Wait all existing dio workers, newcomers will block on i_mutex */
ext4_inode_block_unlocked_dio(inode);
inode_dio_wait(inode);
/*
* Prevent page faults from reinstantiating pages we have
* released from page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
/* Now release the pages and zero block aligned part of pages */
truncate_pagecache_range(inode, start, end - 1);
inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
ret = ext4_alloc_file_blocks(file, lblk, max_blocks, new_size,
flags, mode);
up_write(&EXT4_I(inode)->i_mmap_sem);
if (ret)
goto out_dio;
}
@@ -5524,17 +5518,22 @@ int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)
goto out_mutex;
}
truncate_pagecache(inode, ioffset);
/* Wait for existing dio to complete */
ext4_inode_block_unlocked_dio(inode);
inode_dio_wait(inode);
/*
* Prevent page faults from reinstantiating pages we have released from
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
truncate_pagecache(inode, ioffset);
credits = ext4_writepage_trans_blocks(inode);
handle = ext4_journal_start(inode, EXT4_HT_TRUNCATE, credits);
if (IS_ERR(handle)) {
ret = PTR_ERR(handle);
goto out_dio;
goto out_mmap;
}
down_write(&EXT4_I(inode)->i_data_sem);
@@ -5573,7 +5572,8 @@ int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)
out_stop:
ext4_journal_stop(handle);
out_dio:
out_mmap:
up_write(&EXT4_I(inode)->i_mmap_sem);
ext4_inode_resume_unlocked_dio(inode);
out_mutex:
mutex_unlock(&inode->i_mutex);
@@ -5660,17 +5660,22 @@ int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len)
goto out_mutex;
}
truncate_pagecache(inode, ioffset);
/* Wait for existing dio to complete */
ext4_inode_block_unlocked_dio(inode);
inode_dio_wait(inode);
/*
* Prevent page faults from reinstantiating pages we have released from
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
truncate_pagecache(inode, ioffset);
credits = ext4_writepage_trans_blocks(inode);
handle = ext4_journal_start(inode, EXT4_HT_TRUNCATE, credits);
if (IS_ERR(handle)) {
ret = PTR_ERR(handle);
goto out_dio;
goto out_mmap;
}
/* Expand file to avoid data loss if there is error while shifting */
@@ -5741,7 +5746,8 @@ int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len)
out_stop:
ext4_journal_stop(handle);
out_dio:
out_mmap:
up_write(&EXT4_I(inode)->i_mmap_sem);
ext4_inode_resume_unlocked_dio(inode);
out_mutex:
mutex_unlock(&inode->i_mutex);
View
@@ -209,15 +209,18 @@ static int ext4_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
int result;
handle_t *handle = NULL;
struct super_block *sb = file_inode(vma->vm_file)->i_sb;
struct inode *inode = file_inode(vma->vm_file);
struct super_block *sb = inode->i_sb;
bool write = vmf->flags & FAULT_FLAG_WRITE;
if (write) {
sb_start_pagefault(sb);
file_update_time(vma->vm_file);
down_read(&EXT4_I(inode)->i_mmap_sem);
handle = ext4_journal_start_sb(sb, EXT4_HT_WRITE_PAGE,
EXT4_DATA_TRANS_BLOCKS(sb));
}
} else
down_read(&EXT4_I(inode)->i_mmap_sem);
if (IS_ERR(handle))
result = VM_FAULT_SIGBUS;
@@ -228,8 +231,10 @@ static int ext4_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
if (write) {
if (!IS_ERR(handle))
ext4_journal_stop(handle);
up_read(&EXT4_I(inode)->i_mmap_sem);
sb_end_pagefault(sb);
}
} else
up_read(&EXT4_I(inode)->i_mmap_sem);
return result;
}
@@ -246,10 +251,12 @@ static int ext4_dax_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
if (write) {
sb_start_pagefault(sb);
file_update_time(vma->vm_file);
down_read(&EXT4_I(inode)->i_mmap_sem);
handle = ext4_journal_start_sb(sb, EXT4_HT_WRITE_PAGE,
ext4_chunk_trans_blocks(inode,
PMD_SIZE / PAGE_SIZE));
}
} else
down_read(&EXT4_I(inode)->i_mmap_sem);
if (IS_ERR(handle))
result = VM_FAULT_SIGBUS;
@@ -260,30 +267,71 @@ static int ext4_dax_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
if (write) {
if (!IS_ERR(handle))
ext4_journal_stop(handle);
up_read(&EXT4_I(inode)->i_mmap_sem);
sb_end_pagefault(sb);
}
} else
up_read(&EXT4_I(inode)->i_mmap_sem);
return result;
}
static int ext4_dax_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
{
return dax_mkwrite(vma, vmf, ext4_get_block_dax,
ext4_end_io_unwritten);
int err;
struct inode *inode = file_inode(vma->vm_file);
sb_start_pagefault(inode->i_sb);
file_update_time(vma->vm_file);
down_read(&EXT4_I(inode)->i_mmap_sem);
err = __dax_mkwrite(vma, vmf, ext4_get_block_dax,
ext4_end_io_unwritten);
up_read(&EXT4_I(inode)->i_mmap_sem);
sb_end_pagefault(inode->i_sb);
return err;
}
/*
* Handle write fault for VM_MIXEDMAP mappings. Similarly to ext4_dax_mkwrite()
* handler we check for races agaist truncate. Note that since we cycle through
* i_mmap_sem, we are sure that also any hole punching that began before we
* were called is finished by now and so if it included part of the file we
* are working on, our pte will get unmapped and the check for pte_same() in
* wp_pfn_shared() fails. Thus fault gets retried and things work out as
* desired.
*/
static int ext4_dax_pfn_mkwrite(struct vm_area_struct *vma,
struct vm_fault *vmf)
{
struct inode *inode = file_inode(vma->vm_file);
struct super_block *sb = inode->i_sb;
int ret = VM_FAULT_NOPAGE;
loff_t size;
sb_start_pagefault(sb);
file_update_time(vma->vm_file);
down_read(&EXT4_I(inode)->i_mmap_sem);
size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
if (vmf->pgoff >= size)
ret = VM_FAULT_SIGBUS;
up_read(&EXT4_I(inode)->i_mmap_sem);
sb_end_pagefault(sb);
return ret;
}
static const struct vm_operations_struct ext4_dax_vm_ops = {
.fault = ext4_dax_fault,
.pmd_fault = ext4_dax_pmd_fault,
.page_mkwrite = ext4_dax_mkwrite,
.pfn_mkwrite = dax_pfn_mkwrite,
.pfn_mkwrite = ext4_dax_pfn_mkwrite,
};
#else
#define ext4_dax_vm_ops ext4_file_vm_ops
#endif
static const struct vm_operations_struct ext4_file_vm_ops = {
.fault = filemap_fault,
.fault = ext4_filemap_fault,
.map_pages = filemap_map_pages,
.page_mkwrite = ext4_page_mkwrite,
};
View
@@ -3623,6 +3623,15 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
}
/* Wait all existing dio workers, newcomers will block on i_mutex */
ext4_inode_block_unlocked_dio(inode);
inode_dio_wait(inode);
/*
* Prevent page faults from reinstantiating pages we have released from
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
first_block_offset = round_up(offset, sb->s_blocksize);
last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;
@@ -3631,10 +3640,6 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
truncate_pagecache_range(inode, first_block_offset,
last_block_offset);
/* Wait all existing dio workers, newcomers will block on i_mutex */
ext4_inode_block_unlocked_dio(inode);
inode_dio_wait(inode);
if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
credits = ext4_writepage_trans_blocks(inode);
else
@@ -3680,16 +3685,12 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
if (IS_SYNC(inode))
ext4_handle_sync(handle);
/* Now release the pages again to reduce race window */
if (last_block_offset > first_block_offset)
truncate_pagecache_range(inode, first_block_offset,
last_block_offset);
inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
ext4_mark_inode_dirty(handle, inode);
out_stop:
ext4_journal_stop(handle);
out_dio:
up_write(&EXT4_I(inode)->i_mmap_sem);
ext4_inode_resume_unlocked_dio(inode);
out_mutex:
mutex_unlock(&inode->i_mutex);
@@ -4823,13 +4824,15 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
} else
ext4_wait_for_tail_page_commit(inode);
}
down_write(&EXT4_I(inode)->i_mmap_sem);
/*
* Truncate pagecache after we've waited for commit
* in data=journal mode to make pages freeable.
*/
truncate_pagecache(inode, inode->i_size);
if (shrink)
ext4_truncate(inode);
up_write(&EXT4_I(inode)->i_mmap_sem);
}
if (!rc) {
@@ -5278,6 +5281,8 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
sb_start_pagefault(inode->i_sb);
file_update_time(vma->vm_file);
down_read(&EXT4_I(inode)->i_mmap_sem);
/* Delalloc case is easy... */
if (test_opt(inode->i_sb, DELALLOC) &&
!ext4_should_journal_data(inode) &&
@@ -5347,6 +5352,19 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
out_ret:
ret = block_page_mkwrite_return(ret);
out:
up_read(&EXT4_I(inode)->i_mmap_sem);
sb_end_pagefault(inode->i_sb);
return ret;
}
int ext4_filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
struct inode *inode = file_inode(vma->vm_file);
int err;
down_read(&EXT4_I(inode)->i_mmap_sem);
err = filemap_fault(vma, vmf);
up_read(&EXT4_I(inode)->i_mmap_sem);
return err;
}
View
@@ -958,6 +958,7 @@ static void init_once(void *foo)
INIT_LIST_HEAD(&ei->i_orphan);
init_rwsem(&ei->xattr_sem);
init_rwsem(&ei->i_data_sem);
init_rwsem(&ei->i_mmap_sem);
inode_init_once(&ei->vfs_inode);
}
View
@@ -10,8 +10,10 @@
*/
static inline void ext4_truncate_failed_write(struct inode *inode)
{
down_write(&EXT4_I(inode)->i_mmap_sem);
truncate_inode_pages(inode->i_mapping, inode->i_size);
ext4_truncate(inode);
up_write(&EXT4_I(inode)->i_mmap_sem);
}
/*

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