From: Michal Hocko <mhocko@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>,
Peter Zijlstra <peterz@infradead.org>,
Christoph Hellwig <hch@infradead.org>,
linux-xfs@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
penguin-kernel@I-love.SAKURA.ne.jp
Subject: Re: [PATCH 2/3] xfs: add kmem_alloc_io()
Date: Mon, 26 Aug 2019 14:21:10 +0200 [thread overview]
Message-ID: <20190826122110.GB7659@dhcp22.suse.cz> (raw)
In-Reply-To: <db4a1dae-d69a-0df4-4a71-02c2954ecd75@suse.cz>
On Thu 22-08-19 16:26:42, Vlastimil Babka wrote:
> On 8/22/19 3:17 PM, Dave Chinner wrote:
> > On Thu, Aug 22, 2019 at 02:19:04PM +0200, Vlastimil Babka wrote:
> >> On 8/22/19 2:07 PM, Dave Chinner wrote:
> >> > On Thu, Aug 22, 2019 at 01:14:30PM +0200, Vlastimil Babka wrote:
> >> >
> >> > No, the problem is this (using kmalloc as a general term for
> >> > allocation, whether it be kmalloc, kmem_cache_alloc, alloc_page, etc)
> >> >
> >> > some random kernel code
> >> > kmalloc(GFP_KERNEL)
> >> > reclaim
> >> > PF_MEMALLOC
> >> > shrink_slab
> >> > xfs_inode_shrink
> >> > XFS_ILOCK
> >> > xfs_buf_allocate_memory()
> >> > kmalloc(GFP_KERNEL)
> >> >
> >> > And so locks on inodes in reclaim are seen below reclaim. Then
> >> > somewhere else we have:
> >> >
> >> > some high level read-only xfs code like readdir
> >> > XFS_ILOCK
> >> > xfs_buf_allocate_memory()
> >> > kmalloc(GFP_KERNEL)
> >> > reclaim
> >> >
> >> > And this one throws false positive lockdep warnings because we
> >> > called into reclaim with XFS_ILOCK held and GFP_KERNEL alloc
> >>
> >> OK, and what exactly makes this positive a false one? Why can't it continue like
> >> the first example where reclaim leads to another XFS_ILOCK, thus deadlock?
> >
> > Because above reclaim we only have operations being done on
> > referenced inodes, and below reclaim we only have unreferenced
> > inodes. We never lock the same inode both above and below reclaim
> > at the same time.
> >
> > IOWs, an operation above reclaim cannot see, access or lock
> > unreferenced inodes, except in inode write clustering, and that uses
> > trylocks so cannot deadlock with reclaim.
> >
> > An operation below reclaim cannot see, access or lock referenced
> > inodes except during inode write clustering, and that uses trylocks
> > so cannot deadlock with code above reclaim.
>
> Thanks for elaborating. Perhaps lockdep experts (not me) would know how to
> express that. If not possible, then replacing GFP_NOFS with __GFP_NOLOCKDEP
> should indeed suppress the warning, while allowing FS reclaim.
This was certainly my hope to happen when introducing __GFP_NOLOCKDEP.
I couldn't have done the second step because that requires a deep
understanding of the code in question which is beyond my capacity. It
seems we still haven't found a brave soul to start converting GFP_NOFS
to __GFP_NOLOCKDEP. And it would be really appreciated.
Thanks.
--
Michal Hocko
SUSE Labs
prev parent reply other threads:[~2019-08-26 12:21 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20190821083820.11725-1-david@fromorbit.com>
[not found] ` <20190821083820.11725-3-david@fromorbit.com>
[not found] ` <20190821232440.GB24904@infradead.org>
[not found] ` <20190822003131.GR1119@dread.disaster.area>
2019-08-22 7:59 ` [PATCH 2/3] xfs: add kmem_alloc_io() Christoph Hellwig
2019-08-22 8:51 ` Peter Zijlstra
2019-08-22 9:10 ` Peter Zijlstra
2019-08-22 10:14 ` Dave Chinner
2019-08-22 11:14 ` Vlastimil Babka
2019-08-22 12:07 ` Dave Chinner
2019-08-22 12:19 ` Vlastimil Babka
2019-08-22 13:17 ` Dave Chinner
2019-08-22 14:26 ` Vlastimil Babka
2019-08-26 12:21 ` Michal Hocko [this message]
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