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Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params()

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On Mon 22-05-17 10:52:44, Mikulas Patocka wrote:
> 
> 
> On Mon, 22 May 2017, Michal Hocko wrote:
[...] 
> > I am not sure I understand. OOM killer is invoked for _all_ allocations
> > <= PAGE_ALLOC_COSTLY_ORDER that do not have __GFP_NORETRY as long as the
> > OOM killer is not disabled (oom_killer_disable) and that only happens
> > from the PM suspend path which makes sure that no userspace is active at
> > the time. AFAIU this is a userspace triggered path and so the later
> > shouldn't apply to it and GFP_KERNEL should be therefore sufficient.
> > Relying to a portion of memory reserves to prevent from deadlock seems
> > fundamentaly broken  to me.
> > 
> 
> The lvm2 was designed this way - it is broken, but there is not much that 
> can be done about it - fixing this would mean major rewrite. The only 
> thing we can do about it is to lower the deadlock probability with 
> __GFP_HIGH (or PF_MEMALLOC that was used some times ago).

But let me repeat. GFP_KERNEL allocation for order-0 page will not fail.
If you need non-failing semantic then just make it clear by adding
__GFP_NOFAIL rather than __GFP_HIGH. Memory reserves are a scarce
resource and there are users which might really need it from atomic
contexts.

Anyway, this is not the code I am maintaining so I will not argue more
and won't nack the patch. But is smells like a pure cargo cult, to be
honest.

If you really insist, though, I would just ask to have a more detailed
explanation why it is _believed_ the flag is needed because the vague
"Use __GFP_HIGH to avoid low memory issues when a device is suspended
and the ioctl is needed to resume it." doesn't really clarify much to be
honest.

Thanks!
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Junaid Shahid <junaids@google.com> - 2017-05-19 05:00 +0200
  Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Michal Hocko <mhocko@kernel.org> - 2017-05-19 09:50 +0200
    Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Mikulas Patocka <mpatocka@redhat.com> - 2017-05-20 01:50 +0200
      Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Michal Hocko <mhocko@kernel.org> - 2017-05-22 11:40 +0200
        Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Mikulas Patocka <mpatocka@redhat.com> - 2017-05-22 14:10 +0200
          Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Michal Hocko <mhocko@kernel.org> - 2017-05-22 14:20 +0200
            Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Mikulas Patocka <mpatocka@redhat.com> - 2017-05-22 17:00 +0200
              Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params() Michal Hocko <mhocko@kernel.org> - 2017-05-22 17:10 +0200
                Re: dm ioctl: Restore __GFP_HIGH in copy_params() Mike Snitzer <snitzer@redhat.com> - 2017-05-22 20:10 +0200
                Re: dm ioctl: Restore __GFP_HIGH in copy_params() David Rientjes <rientjes@google.com> - 2017-05-22 22:40 +0200
                Re: dm ioctl: Restore __GFP_HIGH in copy_params() Mike Snitzer <snitzer@redhat.com> - 2017-05-23 01:40 +0200
                Re: dm ioctl: Restore __GFP_HIGH in copy_params() Michal Hocko <mhocko@kernel.org> - 2017-05-23 08:10 +0200
                Re: dm ioctl: Restore __GFP_HIGH in copy_params() Mikulas Patocka <mpatocka@redhat.com> - 2017-05-23 18:50 +0200
                Re: dm ioctl: Restore __GFP_HIGH in copy_params() Michal Hocko <mhocko@kernel.org> - 2017-05-23 09:00 +0200

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