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Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS

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On 2016/04/26 20:56, Michal Hocko wrote:
> Not only this is easier to understand and maintain because there are
> much less problematic contexts than specific allocation requests, this
> also helps code paths where FS layer interacts with other layers (e.g.
> crypto, security modules, MM etc...) and there is no easy way to convey
> the allocation context between the layers.
> 

You arrived at what I wished at
http://lkml.kernel.org/r/201503172305.DIH52162.FOFMFOVJHLOtQS@I-love.SAKURA.ne.jp
(i.e. not CONFIG_DEBUG_* but always enabled).

> Introduce PF_MEMALLOC_NOFS task specific flag and memalloc_nofs_{save,restore}
> API to control the scope. This is basically copying
> memalloc_noio_{save,restore} API we have for other restricted allocation
> context GFP_NOIO.
> 
> Xfs has already had a similar functionality as PF_FSTRANS so let's just
> give it a more generic name and make it usable for others as well and
> move the GFP_NOFS context tracking to the page allocator. Xfs has its
> own accessor functions but let's keep them for now to reduce this patch
> as minimum.
> 
> This patch shouldn't introduce any functional changes. Xfs code paths
> preserve their semantic. kmem_flags_convert() doesn't need to evaluate
> the flag anymore because it is the page allocator to care about the
> flag. memalloc_noio_flags is renamed to current_gfp_context because it
> now cares about both PF_MEMALLOC_NOFS and PF_MEMALLOC_NOIO contexts.
> 
> Let's hope that filesystems will drop direct GFP_NOFS (resp. ~__GFP_FS)
> usage as much and possible and only use a properly documented
> memalloc_nofs_{save,restore} checkpoints where they are appropriate.

Is the story simple enough to monotonically replace GFP_NOFS/GFP_NOIO
with GFP_KERNEL after memalloc_no{fs,io}_{save,restore} are inserted?
We sometimes delegate some operations to somebody else. Don't we need to
convey PF_MEMALLOC_NOFS/PF_MEMALLOC_NOIO flags to APIs which interact with
other threads?

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[PATCH 0/2] scop GFP_NOFS api Michal Hocko <mhocko@kernel.org> - 2016-04-26 14:00 +0200
  [PATCH 2/2] mm, debug: report when GFP_NO{FS,IO} is used explicitly from memalloc_no{fs,io}_{save,restore} context Michal Hocko <mhocko@kernel.org> - 2016-04-26 14:00 +0200
    Re: [PATCH 2/2] mm, debug: report when GFP_NO{FS,IO} is used  explicitly from memalloc_no{fs,io}_{save,restore} context Dave Chinner <david@fromorbit.com> - 2016-04-27 01:00 +0200
      Re: [PATCH 2/2] mm, debug: report when GFP_NO{FS,IO} is used  explicitly from memalloc_no{fs,io}_{save,restore} context Michal Hocko <mhocko@kernel.org> - 2016-04-27 10:10 +0200
        Re: [PATCH 2/2] mm, debug: report when GFP_NO{FS,IO} is used  explicitly from memalloc_no{fs,io}_{save,restore} context Dave Chinner <david@fromorbit.com> - 2016-04-28 01:00 +0200
      Re: [PATCH 2/2] mm, debug: report when GFP_NO{FS,IO} is used  explicitly from memalloc_no{fs,io}_{save,restore} context Michal Hocko <mhocko@kernel.org> - 2016-04-28 10:20 +0200
  [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Michal Hocko <mhocko@kernel.org> - 2016-04-26 14:00 +0200
    Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Dave Chinner <david@fromorbit.com> - 2016-04-27 01:10 +0200
      Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Michal Hocko <mhocko@kernel.org> - 2016-04-27 10:00 +0200
    Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> - 2016-04-27 13:00 +0200
      Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Michal Hocko <mhocko@kernel.org> - 2016-04-27 13:20 +0200
        Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> - 2016-04-27 16:50 +0200
          Re: [PATCH 1/2] mm: add PF_MEMALLOC_NOFS Michal Hocko <mhocko@kernel.org> - 2016-04-27 22:10 +0200
    [PATCH 1.1/2] xfs: abstract PF_FSTRANS to PF_MEMALLOC_NOFS Michal Hocko <mhocko@kernel.org> - 2016-04-27 14:00 +0200
      [PATCH 1.2/2] mm: introduce memalloc_nofs_{save,restore} API Michal Hocko <mhocko@kernel.org> - 2016-04-27 14:00 +0200
        Re: [PATCH 1.2/2] mm: introduce memalloc_nofs_{save,restore} API Michal Hocko <mhocko@kernel.org> - 2016-04-27 15:10 +0200
        Re: [PATCH 1.2/2] mm: introduce memalloc_nofs_{save,restore} API Michal Hocko <mhocko@kernel.org> - 2016-04-27 22:10 +0200
          Re: [PATCH 1.2/2] mm: introduce memalloc_nofs_{save,restore} API Michal Hocko <mhocko@kernel.org> - 2016-04-27 22:40 +0200
        Re: [PATCH 1.2/2] mm: introduce memalloc_nofs_{save,restore} API Michal Hocko <mhocko@kernel.org> - 2016-04-27 23:20 +0200
      Re: [PATCH 1.1/2] xfs: abstract PF_FSTRANS to PF_MEMALLOC_NOFS Andreas Dilger <adilger@dilger.ca> - 2016-04-27 19:50 +0200
        Re: [PATCH 1.1/2] xfs: abstract PF_FSTRANS to PF_MEMALLOC_NOFS Michal Hocko <mhocko@kernel.org> - 2016-04-27 21:50 +0200

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