From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH 4/7] mm: introduce memalloc_nofs_{save,restore} API Date: Mon, 6 Mar 2017 13:22:14 -0800 Message-ID: <20170306132214.1769368301d9e671e1bc68be@linux-foundation.org> References: <20170306131408.9828-1-mhocko@kernel.org> <20170306131408.9828-5-mhocko@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20170306131408.9828-5-mhocko@kernel.org> Sender: owner-linux-mm@kvack.org To: Michal Hocko Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Dave Chinner , djwong@kernel.org, Theodore Ts'o , Chris Mason , David Sterba , Jan Kara , 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 , Michal Hocko , Vlastimil Babka List-Id: ceph-devel.vger.kernel.org On Mon, 6 Mar 2017 14:14:05 +0100 Michal Hocko wrote: > From: Michal Hocko > > 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. > > .... > > --- a/include/linux/gfp.h > +++ b/include/linux/gfp.h > @@ -210,8 +210,16 @@ struct vm_area_struct; > * > * GFP_NOIO will use direct reclaim to discard clean pages or slab pages > * that do not require the starting of any physical IO. > + * Please try to avoid using this flag directly and instead use > + * memalloc_noio_{save,restore} to mark the whole scope which cannot > + * perform any IO with a short explanation why. All allocation requests > + * will inherit GFP_NOIO implicitly. > * > * GFP_NOFS will use direct reclaim but will not use any filesystem interfaces. > + * Please try to avoid using this flag directly and instead use > + * memalloc_nofs_{save,restore} to mark the whole scope which cannot/shouldn't > + * recurse into the FS layer with a short explanation why. All allocation > + * requests will inherit GFP_NOFS implicitly. I wonder if these are worth a checkpatch rule. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org