Messages in this thread Patch in this message |  | | Date | Thu, 20 Sep 2001 01:31:53 +0200 | From | Andrea Arcangeli <> | Subject | Re: pre12 VM doubts and patch |
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On Wed, Sep 19, 2001 at 04:16:13PM -0700, Linus Torvalds wrote: > > On Wed, 19 Sep 2001, Andrea Arcangeli wrote: > > > > On Wed, Sep 19, 2001 at 08:42:39PM +0100, Hugh Dickins wrote: > > > --- 2.4.10-pre12/mm/swap_state.c Wed Sep 19 14:05:54 2001 > > > +++ linux/mm/swap_state.c Mon Sep 17 06:30:26 2001 > > > @@ -23,6 +23,17 @@ > > > */ > > > static int swap_writepage(struct page *page) > > > { > > > + /* One for the page cache, one for this user, one for page->buffers */ > > > + if (page_count(page) > 2 + !!page->buffers) > > > > this is racy, you have to spin_lock(&pagecache_lock) before you can > > expect the page_count() stays constant. then after you checked the page > > has count == 1, you must atomically drop it from the pagecache so it's > > not visible anymore to the swapin lookups. > > No. > > Note how it is a _heuristic_ only. The "safe" answer is always to say "the > page is in use", and note that once the page_count has dropped to 2 or > less, it won't increase unless somebody else has a swap count..
the "somebody else has a swap count" is interesting. so we rely on the fact any swap_duplicate is always run before the swapcache is unlocked.
Also the "> 2" should be "> 1", but really I noticed I cannot avoid getting a reference so please apply also this patch instead of replacing "> 2" with "> 1" (it was a race condition):
--- 2.4.10pre11aa1/mm/vmscan.c.~1~ Tue Sep 18 21:23:49 2001 +++ 2.4.10pre11aa1/mm/vmscan.c Thu Sep 20 01:29:58 2001 @@ -415,7 +415,10 @@ spin_unlock(&pagemap_lru_lock); ClearPageDirty(page); + + page_cache_get(page); writepage(page); + page_cache_release(page); spin_lock(&pagemap_lru_lock); continue;
> And we check for the "somebody else has a swap count" two lines lower.
I see.
> Do you see anything wrong with that logic?
Looks ok now :), I guess it would be good to write a comment now, so maybe other people won't share my worry, the swap_count thing wasn't very obvious. thanks,
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