linux-ext4 - Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

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Date:   Wed, 14 Dec 2016 10:05:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Anatoly Stepanov <astepanov@...udlinux.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Theodore Ts'o <tytso@....edu>, kvm@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-security-module@...r.kernel.org,
        Dave Chinner <david@...morbit.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

On Tue 13-12-16 13:55:46, Andreas Dilger wrote:
> On Dec 13, 2016, at 3:14 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > 
> > Are there any more comments or objections to this patch? Is this a good
> > start or kv[mz]alloc has to provide a way to cover GFP_NOFS users as
> > well in the initial version.
> 
> I'm in favour of this cleanup as a starting point.  I definitely agree
> that this same functionality is in use in a number of places and should
> be consolidated.
> 
> The vmalloc() from GFP_NOFS can be addressed separately in later patches.
> That is an issue for several filesystems, and while XFS works around this,
> it would be better to lift that out of the filesystem code into the VM.

Well, my longer term plan is to change how GFP_NOFS is used from the fs
code rather than tweak the VM layer. The current situation with the nofs
is messy and confusing. In many contexts it is used without a good
reason - just to be sure that nothing will break. I strongly believe
that we should use a scope api [1] which marks whole regions of
potentially reclaim dangerous code paths and all the allocations within
that region will inherit the nofs protection automatically. That would
solve the vmalloc(GFP_NOFS) problem as well. The route to get there is
no short or easy. I am planning to repost the scope patchset hopefully
soon with ext4 converted.

[1] http://lkml.kernel.org/r/1461671772-1269-1-git-send-email-mhocko@kernel.org

> Really, there are several of things about vmalloc() that could improve
> if we decided to move it out of the dog house and allow it to become a
> first class citizen, but that needs a larger discussion, and you can
> already do a lot of cleanup with just the introduction of kvmalloc().
> 
> Since this is changing the ext4 code, you can add my:
> 
> Reviewed-by: Andreas Dilger <adilger@...ger.ca>

thanks!
-- 
Michal Hocko
SUSE Labs
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