Re: [PATCH] mm, page_alloc: Remove debug_guardpage_minorder() test in warn_alloc().

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Wed, Apr 12, 2017 at 12:59:51PM +0200, Michal Hocko wrote:
> On Wed 12-04-17 12:23:45, Stanislaw Gruszka wrote:
> > On Tue, Apr 11, 2017 at 08:27:15PM +0900, Tetsuo Handa wrote:
> > > We are using warn_alloc() for reporting both allocation failures and
> > > allocation stalls. If we add debug_guardpage_minorder=1 parameter,
> > > all allocation failure and allocation stall reports become pointless
> > > like below. (Below output would be an OOM livelock were all __GFP_FS
> > > allocations got stuck at too_many_isolated() in shrink_inactive_list()
> > > waiting for kswapd, kswapd is waiting for !__GFP_FS allocations, and
> > > all !__GFP_FS allocations did not get stuck at too_many_isolated() in
> > > shrink_inactive_list() but are unable to invoke the OOM killer.)
> > > 
> > > ----------
> > > [    0.000000] Linux version 4.11.0-rc6-next-20170410 (root@ccsecurity) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC) ) #578 SMP Mon Apr 10 23:08:53 JST 2017
> > > [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.11.0-rc6-next-20170410 (...snipped...) debug_guardpage_minorder=1
> > > (...snipped...)
> > > [    0.000000] Setting debug_guardpage_minorder to 1
> > > (...snipped...)
> > > [   99.064207] Out of memory: Kill process 3097 (a.out) score 999 or sacrifice child
> > > [   99.066488] Killed process 3097 (a.out) total-vm:14408kB, anon-rss:84kB, file-rss:36kB, shmem-rss:0kB
> > > [   99.180378] oom_reaper: reaped process 3097 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> > > [  128.310487] warn_alloc: 266 callbacks suppressed
> > > [  133.445395] warn_alloc: 74 callbacks suppressed
> > > [  138.517471] warn_alloc: 300 callbacks suppressed
> > > [  143.537630] warn_alloc: 34 callbacks suppressed
> > > [  148.610773] warn_alloc: 277 callbacks suppressed
> > > [  153.630652] warn_alloc: 70 callbacks suppressed
> > > [  158.639891] warn_alloc: 217 callbacks suppressed
> > > [  163.687727] warn_alloc: 120 callbacks suppressed
> > > [  168.709610] warn_alloc: 252 callbacks suppressed
> > > [  173.714659] warn_alloc: 103 callbacks suppressed
> > > [  178.730858] warn_alloc: 248 callbacks suppressed
> > > [  183.797587] warn_alloc: 82 callbacks suppressed
> > > [  188.825250] warn_alloc: 238 callbacks suppressed
> > > [  193.832834] warn_alloc: 102 callbacks suppressed
> > > [  198.876409] warn_alloc: 259 callbacks suppressed
> > > [  203.940073] warn_alloc: 102 callbacks suppressed
> > > [  207.620979] sysrq: SysRq : Resetting
> > > ----------
> > > 
> > > Commit c0a32fc5a2e470d0 ("mm: more intensive memory corruption debugging")
> > > changed to check debug_guardpage_minorder() > 0 when reporting allocation
> > > failures. But the patch description seems to lack why we want to check it.
> > 
> > When we use guard page to debug memory corruption, it shrinks available
> > pages to 1/2, 1/4, 1/8 and so on, depending on parameter value.
> > In such case memory allocation failures can be common and printing
> > errors can flood dmesg. If sombody debug corruption, allocation
> > failures are not the things he/she is interested about.
> 
> Can we distinguish those guard page allocations?

Allocation failures happen on standard pages, due to limit of available pages.
Because much of pages become unused - guard pages are reserved pages marked
as no-read/no-write (basically this is artificial memory shrink).

>Why cannot they use
> __GFP_NOWARN?

That some option, though I think setting __GFP_NOWARN if debug_guardpage_enabled()
is set, instead of checking that directly make no big difference anyway.

Stanislaw

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