From: Vlastimil Babka <vbabka@suse.cz>
To: Michal Hocko <mhocko@kernel.org>,
linux-mm@kvack.org, linux-fsdevel@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
Dave Chinner <david@fromorbit.com>,
djwong@kernel.org, "Theodore Ts'o" <tytso@mit.edu>,
Chris Mason <clm@fb.com>, David Sterba <dsterba@suse.cz>,
Jan Kara <jack@suse.cz>,
ceph-devel@vger.kernel.org, cluster-devel@redhat.com,
linux-nfs@vger.kernel.org, logfs@logfs.org,
linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-btrfs@vger.kernel.org, linux-mtd@lists.infradead.org,
reiserfs-devel@vger.kernel.org,
linux-ntfs-dev@lists.sourceforge.net,
linux-f2fs-devel@lists.sourceforge.net,
linux-afs@lists.infradead.org,
LKML <linux-kernel@vger.kernel.org>,
Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 3/8] mm: introduce memalloc_nofs_{save,restore} API
Date: Mon, 9 Jan 2017 14:04:21 +0100 [thread overview]
Message-ID: <86dbce74-a532-2f98-6a63-4dbad77b2aa1@suse.cz> (raw)
In-Reply-To: <20170106141107.23953-4-mhocko@kernel.org>
On 01/06/2017 03:11 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> GFP_NOFS context is used for the following 5 reasons currently
> - to prevent from deadlocks when the lock held by the allocation
> context would be needed during the memory reclaim
> - to prevent from stack overflows during the reclaim because
> the allocation is performed from a deep context already
> - to prevent lockups when the allocation context depends on
> other reclaimers to make a forward progress indirectly
> - just in case because this would be safe from the fs POV
> - silence lockdep false positives
>
> Unfortunately overuse of this allocation context brings some problems
> to the MM. Memory reclaim is much weaker (especially during heavy FS
> metadata workloads), OOM killer cannot be invoked because the MM layer
> doesn't have enough information about how much memory is freeable by the
> FS layer.
>
> In many cases it is far from clear why the weaker context is even used
> and so it might be used unnecessarily. We would like to get rid of
> those as much as possible. One way to do that is to use the flag in
> scopes rather than isolated cases. Such a scope is declared when really
> necessary, tracked per task and all the allocation requests from within
> the context will simply inherit the GFP_NOFS semantic.
>
> 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.
>
> Introduce memalloc_nofs_{save,restore} API to control the scope
> of GFP_NOFS allocation context. This is basically copying
> memalloc_noio_{save,restore} API we have for other restricted allocation
> context GFP_NOIO. The PF_MEMALLOC_NOFS flag already exists and it is
> just an alias for PF_FSTRANS which has been xfs specific until recently.
> There are no more PF_FSTRANS users anymore so let's just drop it.
>
> PF_MEMALLOC_NOFS is now checked in the MM layer and drops __GFP_FS
> implicitly same as PF_MEMALLOC_NOIO drops __GFP_IO. memalloc_noio_flags
> is renamed to current_gfp_context because it now cares about both
> PF_MEMALLOC_NOFS and PF_MEMALLOC_NOIO contexts. Xfs code paths preserve
> their semantic. kmem_flags_convert() doesn't need to evaluate the flag
> anymore.
>
> This patch shouldn't introduce any functional changes.
>
> Let's hope that filesystems will drop direct GFP_NOFS (resp. ~__GFP_FS)
> usage as much as possible and only use a properly documented
> memalloc_nofs_{save,restore} checkpoints where they are appropriate.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
[...]
> +static inline unsigned int memalloc_nofs_save(void)
> +{
> + unsigned int flags = current->flags & PF_MEMALLOC_NOFS;
> + current->flags |= PF_MEMALLOC_NOFS;
So this is not new, as same goes for memalloc_noio_save, but I've
noticed that e.g. exit_signal() does tsk->flags |= PF_EXITING;
So is it possible that there's a r-m-w hazard here?
> + return flags;
> +}
> +
> +static inline void memalloc_nofs_restore(unsigned int flags)
> +{
> + current->flags = (current->flags & ~PF_MEMALLOC_NOFS) | flags;
> +}
> +
> /* Per-process atomic flags. */
> #define PFA_NO_NEW_PRIVS 0 /* May not gain new privileges. */
> #define PFA_SPREAD_PAGE 1 /* Spread page cache over cpuset */
[...]
> @@ -3029,7 +3029,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> int nid;
> struct scan_control sc = {
> .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> - .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
> + .gfp_mask = (current_gfp_context(gfp_mask) & GFP_RECLAIM_MASK) |
So this function didn't do memalloc_noio_flags() before? Is it a bug
that should be fixed separately or at least mentioned? Because that
looks like a functional change...
Thanks!
> (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
> .reclaim_idx = MAX_NR_ZONES - 1,
> .target_mem_cgroup = memcg,
> @@ -3723,7 +3723,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> int classzone_idx = gfp_zone(gfp_mask);
> struct scan_control sc = {
> .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> - .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
> + .gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
> .order = order,
> .priority = NODE_RECLAIM_PRIORITY,
> .may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
>
next prev parent reply other threads:[~2017-01-09 13:04 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-06 14:10 [PATCH 0/8 v3] scope GFP_NOFS api Michal Hocko
2017-01-06 14:11 ` [PATCH 1/8] lockdep: allow to disable reclaim lockup detection Michal Hocko
2017-01-09 12:56 ` Vlastimil Babka
2017-01-06 14:11 ` [PATCH 2/8] xfs: abstract PF_FSTRANS to PF_MEMALLOC_NOFS Michal Hocko
2017-01-09 12:59 ` Vlastimil Babka
2017-01-09 14:29 ` Michal Hocko
2017-01-09 20:58 ` Darrick J. Wong
2017-01-06 14:11 ` [PATCH 3/8] mm: introduce memalloc_nofs_{save,restore} API Michal Hocko
2017-01-09 13:04 ` Vlastimil Babka [this message]
2017-01-09 13:42 ` Michal Hocko
2017-01-09 13:59 ` Michal Hocko
2017-01-09 14:04 ` Vlastimil Babka
2017-01-06 14:11 ` [PATCH 4/8] xfs: use memalloc_nofs_{save,restore} instead of memalloc_noio* Michal Hocko
2017-01-09 14:08 ` Vlastimil Babka
2017-01-09 14:25 ` Michal Hocko
2017-01-09 15:56 ` Brian Foster
2017-01-09 20:59 ` Darrick J. Wong
2017-01-06 14:11 ` [PATCH 5/8] jbd2: mark the transaction context with the scope GFP_NOFS context Michal Hocko
2017-01-06 14:11 ` [PATCH 6/8] jbd2: make the whole kjournald2 kthread NOFS safe Michal Hocko
2017-01-06 14:11 ` [PATCH 7/8] Revert "ext4: avoid deadlocks in the writeback path by using sb_getblk_gfp" Michal Hocko
2017-01-17 3:01 ` Theodore Ts'o
2017-01-17 7:54 ` Michal Hocko
2017-03-06 11:59 ` Michal Hocko
2017-01-06 14:11 ` [PATCH 8/8] Revert "ext4: fix wrong gfp type under transaction" Michal Hocko
2017-01-17 2:56 ` Theodore Ts'o
2017-01-17 8:24 ` Michal Hocko
2017-01-17 15:18 ` Michal Hocko
2017-01-17 15:59 ` Theodore Ts'o
2017-01-17 16:16 ` Michal Hocko
2017-01-17 17:29 ` Jan Kara
2017-01-19 8:39 ` Michal Hocko
2017-01-19 9:22 ` Jan Kara
2017-01-19 9:44 ` Michal Hocko
2017-01-26 7:44 ` Michal Hocko
2017-01-27 6:13 ` Theodore Ts'o
2017-01-27 9:37 ` Michal Hocko
2017-01-27 16:40 ` Theodore Ts'o
2017-01-28 7:32 ` [Cluster-devel] " Christoph Hellwig
2017-01-28 8:17 ` David Lang
2017-01-30 8:12 ` Michal Hocko
2017-02-03 15:32 ` Michal Hocko
2017-01-17 21:04 ` Andreas Dilger
2017-01-18 8:29 ` Michal Hocko
2017-01-06 14:18 ` [DEBUG PATCH 0/2] debug explicit GFP_NO{FS,IO} usage from the scope context Michal Hocko
2017-01-06 14:18 ` [DEBUG PATCH 1/2] mm, debug: report when GFP_NO{FS,IO} is used explicitly from memalloc_no{fs,io}_{save,restore} context Michal Hocko
2017-01-06 14:18 ` [DEBUG PATCH 2/2] silent warnings which we cannot do anything about Michal Hocko
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