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

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Message-ID: <20161209020016.GX1555@ZenIV.linux.org.uk>
Date:   Fri, 9 Dec 2016 02:00:17 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Dave Chinner <david@...morbit.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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>,
        Michal Hocko <mhocko@...e.com>,
        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
Subject: Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

On Fri, Dec 09, 2016 at 12:44:17PM +1100, Dave Chinner wrote:
> On Thu, Dec 08, 2016 at 11:33:00AM +0100, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > Using kmalloc with the vmalloc fallback for larger allocations is a
> > common pattern in the kernel code. Yet we do not have any common helper
> > for that and so users have invented their own helpers. Some of them are
> > really creative when doing so. Let's just add kv[mz]alloc and make sure
> > it is implemented properly. This implementation makes sure to not make
> > a large memory pressure for > PAGE_SZE requests (__GFP_NORETRY) and also
> > to not warn about allocation failures. This also rules out the OOM
> > killer as the vmalloc is a more approapriate fallback than a disruptive
> > user visible action.
> > 
> > This patch also changes some existing users and removes helpers which
> > are specific for them. In some cases this is not possible (e.g.
> > ext4_kvmalloc, libcfs_kvzalloc, __aa_kvmalloc) because those seems to be
> > broken and require GFP_NO{FS,IO} context which is not vmalloc compatible
> > in general (note that the page table allocation is GFP_KERNEL). Those
> > need to be fixed separately.
> 
> See fs/xfs/kmem.c::kmem_zalloc_large(), which is XFS's version of
> kvmalloc() that is GFP_NOFS/GFP_NOIO safe. Any generic API for this
> functionality will have to play these memalloc_noio_save/
> memalloc_noio_restore games to ensure they are GFP_NOFS safe....

Easier to handle those in vmalloc() itself.  The problem I have with these
helpers is that different places have different cutoff thresholds for
switch from kmalloc to vmalloc; has anyone done an analysis of those?
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