Re: [PATCHv4 00/25] THP-enabled tmpfs/shmem

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Mar 25, 2016 at 05:00:50PM -0700, Hugh Dickins wrote:
> On Fri, 25 Mar 2016, Kirill A. Shutemov wrote:
> > On Thu, Mar 24, 2016 at 12:08:55PM -0700, Hugh Dickins wrote:
> > > On Thu, 24 Mar 2016, Kirill A. Shutemov wrote:
> > > > On Wed, Mar 23, 2016 at 01:09:05PM -0700, Hugh Dickins wrote:
> > > > > The small files thing formed my first impression.  My second
> > > > > impression was similar, when I tried mmap(NULL, size_of_RAM,
> > > > > PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, -1, 0) and
> > > > > cycled around the arena touching all the pages (which of
> > > > > course has to push a little into swap): that soon OOMed.
> > > > > 
> > > > > But there I think you probably just have some minor bug to be fixed:
> > > > > I spent a little while trying to debug it, but then decided I'd
> > > > > better get back to writing to you.  I didn't really understand what
> > > > > I was seeing, but when I hacked some stats into shrink_page_list(),
> > > > > converting !is_page_cache_freeable(page) to page_cache_references(page)
> > > > > to return the difference instead of the bool, a large proportion of
> > > > > huge tmpfs pages seemed to have count 1 too high to be freeable at
> > > > > that point (and one huge tmpfs page had a count of 3477).
> > > > 
> > > > I'll reply to your other points later, but first I wanted to address this
> > > > obvious bug.
> > > 
> > > Thanks.  That works better, but is not yet right: memory isn't freed
> > > as it should be, so when I exit then try to run a second time, the
> > > mmap() just gets ENOMEM (with /proc/sys/vm/overcommit_memory 0):
> > > MemFree is low.  No rush to fix, I've other stuff to do.
> > > 
> > > I don't get as far as that on the laptop, since the first run is OOM
> > > killed while swapping; but I can't vouch for the OOM-kill-correctness
> > > of the base tree I'm using, and this laptop has a history of OOMing
> > > rather too easily if all's not right.
> > 
> > Hm. I don't see the issue.
> > 
> > I tried to reproduce it in my VM with following script:
> > 
> > #!/bin/sh -efu
> > 
> > swapon -a
> > 
> > ram="$(grep MemTotal /proc/meminfo | sed 's,[^0-9\]\+,,; s, kB,k,')"
> > 
> > usemem -w -f /dev/zero "$ram"
> > 
> > swapoff -a
> > swapon -a
> > 
> > usemem -w -f /dev/zero "$ram"
> > 
> > cat /proc/meminfo
> > grep thp /proc/vmstat
> > 
> > -----
> > 
> > usemem is a tool from this archive:
> > 
> > http://www.spinics.net/lists/linux-mm/attachments/gtarazbJaHPaAT.gtar
> > 
> > It works fine even if would double size of mapping.
> > 
> > Do you have a reproducer?
> 
> Yes, my reproducer is simpler (just cycling twice around the arena,
> touching each page in order); and I too did not see it running your
> script using usemem above.  It looks as if that invocation isn't doing
> enough work with swap: if I add a "-r 2" to those usemem lines, then
> I get "usemem: mmap failed: Cannot allocate memory" on the second.
> 
> I also added a "sleep 2" before the second call to usemem: I'm not sure
> of the current state of vmstat, but historically it's slow to gather
> back from each cpu to global, and I think it used to leave some cpu
> counts stranded indefinitely once upon a time.  In my own testing,
> I have a /proc/sys/vm/stat_refresh to touch before checking meminfo
> or vmstat - and I think the vm_enough_memory() check in mmap() may
> need that same care, since it refers to NR_FREE_PAGES etc.
> 
> 8GB is my ramsize, if that matters.

I think I found it. I have refcounting screwed up in faultaround.

This should fix the problem:

diff --git a/mm/filemap.c b/mm/filemap.c
index 94c097ec08e7..1325bb4568d1 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2292,19 +2292,18 @@ repeat:
 		if (fe->pte)
 			fe->pte += iter.index - last_pgoff;
 		last_pgoff = iter.index;
-		alloc_set_pte(fe, NULL, page);
+		if (alloc_set_pte(fe, NULL, page))
+			goto unlock;
 		unlock_page(page);
-		/* Huge page is mapped? No need to proceed. */
-		if (pmd_trans_huge(*fe->pmd))
-			break;
-		/* Failed to setup page table? */
-		VM_BUG_ON(!fe->pte);
 		goto next;
 unlock:
 		unlock_page(page);
 skip:
 		page_cache_release(page);
 next:
+		/* Huge page is mapped? No need to proceed. */
+		if (pmd_trans_huge(*fe->pmd))
+			break;
 		if (iter.index == end_pgoff)
 			break;
 	}
-- 
 Kirill A. Shutemov

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