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cephfs: potential adjust failure in lru_expire #19277

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merged 1 commit into from Dec 18, 2017
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cephfs: potential adjust failure in lru_expire #19277

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3 changes: 1 addition & 2 deletions src/include/lru.h
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Expand Up @@ -157,14 +157,14 @@ class LRU {

// expire -- expire a single item
LRUObject *lru_get_next_expire() {
adjust();
// look through tail of bot
while (bottom.size()) {
LRUObject *p = bottom.back();
if (!p->lru_pinned) return p;

// move to pintail
pintail.push_front(&p->lru_link);
adjust();
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I believe this adjust() is necessary because the size of bottom is changing so it may be necessary to move objects from top to bottom.

}

// ok, try head then
Expand All @@ -174,7 +174,6 @@ class LRU {

// move to pintail
pintail.push_front(&p->lru_link);
adjust();
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This loop probably isn't well tested because bottom will be empty if we are in this loop. I think you're right it may be possible for adjust() to crash if we remove an item from top here. Is that the problem you found originally? If that's the case, then we should fix adjust() to handle bottom.size() == 0, not remove this call here.

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int adjust():
uint64_t toplen = top.size();
uint64_t topwant = (midpoint * (double)(lru_get_size() - num_pinned));

both lru_get_size() and num_pinned do not change when moving item to pintail

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Great point. Thanks Zheng.

}

// no luck!
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