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filemap: add a comment about FAULT_FLAG_RETRY_NOWAIT behavior

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I thought Josef Bacik's patch to drop the mmap_sem was buggy, because
when looking at the error cases, there was one case where we returned
VM_FAULT_RETRY without actually dropping the mmap_sem.

Josef had to explain to me (using small words) that yes, that's actually
what we're supposed to do, and his patch was correct.  Which not only
convinced me he knew what he was doing and I should stop arguing with
him, but also that I should add a comment to the case I was confused
about.

Patiently-pointed-out-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2019-03-15 11:26:07 -07:00
parent 6b4c9f4469
commit 8b0f9fa2e0
1 changed files with 5 additions and 0 deletions
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5
mm/filemap.c
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@ -2428,6 +2428,11 @@ static int lock_page_maybe_drop_mmap(struct vm_fault *vmf, struct page *page,
if (trylock_page(page))
return 1;
/*
* NOTE! This will make us return with VM_FAULT_RETRY, but with
* the mmap_sem still held. That's how FAULT_FLAG_RETRY_NOWAIT
* is supposed to work. We have way too many special cases..
*/
if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
return 0;