linux-kernel - Re: [PATCH] btrfs: fix allocation of bitmap pages.

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Date:   Tue, 20 Aug 2019 18:48:58 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Hellwig <hch@...radead.org>, dsterba@...e.cz,
        Christophe Leroy <christophe.leroy@....fr>,
        erhard_f@...lbox.org, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-btrfs@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] btrfs: fix allocation of bitmap pages.

On Tue, Aug 20, 2019 at 01:06:25PM +0200, Vlastimil Babka wrote:
> > The whole point of copy_page is to copy exactly one page and it makes
> > sense to assume that is aligned.  A sane memcpy would use the same
> > underlying primitives as well after checking they fit.  So I think the
> > prime issue here is btrfs' use of copy_page instead of memcpy.  The
> > secondary issue is slub fucking up alignments for no good reason.  We
> > just got bitten by that crap again in XFS as well :(
> 
> Meh, I should finally get back to https://lwn.net/Articles/787740/ right

Yes.  For now Dave came up with an idea for a workaround that will
be forward-compatible with that:

https://www.spinics.net/lists/linux-xfs/msg30521.html