linux-kernel - Re: [PATCH] bcachefs: Switch to memalloc_flags

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Message-ID: <CALOAHbD=mzSBoNqCVf5TTOge4oTZq7Foxdv4H2U1zfBwjNoVKA@mail.gmail.com>
Date: Tue, 3 Sep 2024 14:34:05 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Dave Chinner <david@...morbit.com>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH] bcachefs: Switch to memalloc_flags_do() for vmalloc allocations

On Mon, Sep 2, 2024 at 5:09 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 02-09-24 17:01:12, Yafang Shao wrote:
> > > I really do not see why GFP_NOFAIL should be any special in this
> > > specific case.
> >
> > I believe there's no way to stop it from looping, even if you
> > implement a sophisticated user space OOM killer. ;)
>
> User space OOM killer should be helping to replenish a free memory and
> we have some heuristics to help NOFAIL users out with some portion of
> memory reserves already IIRC. So we do already give them some special
> treatment in the page allocator path. Not so much in the reclaim path.

When setting GFP_NOFAIL, it's important to not only enable direct
reclaim but also the OOM killer. In scenarios where swap is off and
there is minimal page cache, setting GFP_NOFAIL without __GFP_FS can
result in an infinite loop. In other words, GFP_NOFAIL should not be
used with GFP_NOFS. Unfortunately, many call sites do combine them.
For example:

XFS:

fs/xfs/libxfs/xfs_exchmaps.c: GFP_NOFS | __GFP_NOFAIL
fs/xfs/xfs_attr_item.c: GFP_NOFS | __GFP_NOFAIL

EXT4:

fs/ext4/mballoc.c: GFP_NOFS | __GFP_NOFAIL
fs/ext4/extents.c: GFP_NOFS | __GFP_NOFAIL

This seems problematic, but I'm not an FS expert. Perhaps Dave or Ted
could provide further insight.

--
Regards

Yafang