Re: [PATCH] mm: don't account shared file pages in user_reserve_pages

[Konstantin Khlebnikov]

On 29.01.2015 23:11, Andrew Shewmaker wrote:
> On Thu, Jan 29, 2015 at 02:51:27PM +0300, Roman Gushchin wrote:
> > Shared file pages are never accounted in memory overcommit code,
> > so it isn't reasonable to count them in a code that limits the
> > maximal size of a process in OVERCOMMIT_NONE mode.
> >
> > If a process has few large file mappings, the consequent attempts
> > to allocate anonymous memory may unexpectedly fail with -ENOMEM,
> > while there is free memory and overcommit limit if significantly
> > larger than the committed amount (as displayed in /proc/meminfo).
> >
> > The problem is significantly smoothed by commit c9b1d0981fcc
> > ("mm: limit growth of 3% hardcoded other user reserve"),
> > which limits the impact of this check with 128Mb (tunable via sysctl),
> > but it can still be a problem on small machines.
> >
> > Signed-off-by: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Andrew Shewmaker <agshew@xxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > ---
> >   mm/mmap.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 7f684d5..151fadf 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -220,7 +220,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
> >   	 */
> >   	if (mm) {
> >   		reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);
> > -		allowed -= min(mm->total_vm / 32, reserve);
> > +		allowed -= min((mm->total_vm - mm->shared_vm) / 32, reserve);
> >   	}
> >
> >   	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
> > --
> > 2.1.0
>
> You're two patches conflict, don't they? Maybe you should resend
> them as a patch series such that they can both be applied?

I think arithmetic overflow is more important. Upper bound 128M
for user reserve makes mis-accounting of shared memory mostly invisible.

> Does mm->shared_vm include memory that's mapped MAP_ANONYMOUS in
> conjunction with MAP_SHARED? If so, then subtracting it could
> overcommit the system OVERCOMMIT_NEVER mode.

Yep.

Moreover shared_vm also includes file mappings with MAP_PRIVATE.
It works more likely as "maybe shared", upper bound for "file-rss"
(MM_FILEPAGES).

I think we need here total size of vmas where VM_ACCOUNT is set --
writable private mappings mapped without MAP_NORESERVE or something
like that. But total_vm after limiting with 128Mb gives almost always
the same or similar value. So, let's keep it as is.

--
Konstantin

> -Andrew

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