Re: [PATCH 05/18] mm: memcontrol: convert page cache to a new mem_cgroup_charge() API

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, May 11, 2020 at 12:38:04AM -0700, Hugh Dickins wrote:
> On Fri, 8 May 2020, Johannes Weiner wrote:
> > 
> > I looked at this some more, as well as compared it to non-shmem
> > swapping. My conclusion is - and Hugh may correct me on this - that
> > the deletion looks mandatory but is actually an optimization. Page
> > reclaim will ultimately pick these pages up.
> > 
> > When non-shmem pages are swapped in by readahead (locked until IO
> > completes) and their page tables are simultaneously unmapped, the
> > zap_pte_range() code calls free_swap_and_cache() and the locked pages
> > are stranded in the swap cache with no page table references. We rely
> > on page reclaim to pick them up later on.
> > 
> > The same appears to be true for shmem. If the references to the swap
> > page are zapped while we're trying to swap in, we can strand the page
> > in the swap cache. But it's not up to swapin to detect this reliably,
> > it just frees the page more quickly than having to wait for reclaim.
> 
> I think you've got all that exactly right, thanks for working it out.
> It originates from v3.7's 215c02bc33bb ("tmpfs: fix shmem_getpage_gfp()
> VM_BUG_ON") - in which I also had to thank you.

I should have looked where it actually came from - I had forgotten
about that patch!

> I think I chose to do the delete_from_swap_cache() right there, partly
> because of following shmem_unuse_inode() code which already did that,
> partly on the basis that while we have to observe the case then it's
> better to clean it up, and partly out of guilt that our page lock here
> is what had prevented shmem_undo_range() from completing its job; but
> I believe you're right that unused swapcache reclaim would sort it out
> eventually.

That makes sense to me.

> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index e80167927dce..236642775f89 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -640,7 +640,7 @@ static int shmem_add_to_page_cache(struct page *page,
> >  		xas_lock_irq(&xas);
> >  		entry = xas_find_conflict(&xas);
> >  		if (entry != expected)
> > -			xas_set_err(&xas, -EEXIST);
> > +			xas_set_err(&xas, expected ? -ENOENT : -EEXIST);
> 
> Two things on this.
> 
> Minor matter of taste, I'd prefer that as
> 			xas_set_err(&xas, entry ? -EEXIST : -ENOENT);
> which would be more general and more understandable -
> but what you have written should be fine for the actual callers.

Yes, checking `expected' was to differentiate the behavior depending
on the callsite. But testing `entry' is more obvious in that location.

> Except... I think returning -ENOENT there will not work correctly,
> in the case of a punched hole.  Because (unless you've reworked it
> and I just haven't looked) shmem_getpage_gfp() knows to retry in
> the case of -EEXIST, but -ENOENT will percolate up to shmem_fault()
> and result in a SIGBUS, or a read/write error, when the hole should
> just get refilled instead.

Good catch, I had indeed missed that. I'm going to make it retry on
-ENOENT as well.

We could have it go directly to allocating a new page, but it seems
unnecessarily complicated: we've already been retrying in this
situation until now, so I would stick to "there was a race, retry."

> Not something that needs fixing in a hurry (it took trinity to
> generate this racy case in the first place), I'll take another look
> once I've pulled it into a tree (or collected next mmotm) - unless
> you've already have changed it around by then.

Attaching a delta fix based on your observations.

Andrew, barring any objections to this, could you please fold it into
the version you have in your tree already?

---

From 33d03ceebce0a6261d472ddc9c5a07940f44714c Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Mon, 11 May 2020 10:45:14 -0400
Subject: [PATCH] mm: memcontrol: convert page cache to a new
 mem_cgroup_charge() API fix

Incorporate Hugh's feedback:

- shmem_getpage_gfp() needs to handle the new -ENOENT that was
  previously implied in the -EEXIST when a swap entry changed under us
  in any way. Otherwise hole punching could cause a racing fault to
  SIGBUS instead of allocating a new page.

- It is indeed page reclaim that picks up any swapcache we leave
  stranded when free_swap_and_cache() runs on a page locked by
  somebody else. Document that our delete_from_swap_cache() is an
  optimization, not something we rely on for correctness.

- Style cleanup: testing `expected' to decide on -EEXIST vs -ENOENT
  differentiates the callsites, but is a bit awkward to read. Test
  `entry' instead.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/shmem.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index afd5a057ebb7..00fb001e8f3e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -638,7 +638,7 @@ static int shmem_add_to_page_cache(struct page *page,
 		xas_lock_irq(&xas);
 		entry = xas_find_conflict(&xas);
 		if (entry != expected)
-			xas_set_err(&xas, expected ? -ENOENT : -EEXIST);
+			xas_set_err(&xas, entry ? -EEXIST : -ENOENT);
 		xas_create_range(&xas);
 		if (xas_error(&xas))
 			goto unlock;
@@ -1686,10 +1686,13 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 		 * We already confirmed swap under page lock, but
 		 * free_swap_and_cache() only trylocks a page, so it
 		 * is just possible that the entry has been truncated
-		 * or holepunched since swap was confirmed.
-		 * shmem_undo_range() will have done some of the
-		 * unaccounting, now delete_from_swap_cache() will do
-		 * the rest.
+		 * or holepunched since swap was confirmed. This could
+		 * occur at any time while the page is locked, and
+		 * usually page reclaim will take care of the stranded
+		 * swapcache page. But when we catch it, we may as
+		 * well clean up after ourselves: shmem_undo_range()
+		 * will have done some of the unaccounting, now
+		 * delete_from_swap_cache() will do the rest.
 		 */
 		if (error == -ENOENT)
 			delete_from_swap_cache(page);
@@ -1765,7 +1768,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	if (xa_is_value(page)) {
 		error = shmem_swapin_page(inode, index, &page,
 					  sgp, gfp, vma, fault_type);
-		if (error == -EEXIST)
+		if (error == -EEXIST || error == -ENOENT)
 			goto repeat;
 
 		*pagep = page;
-- 
2.26.2