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

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Message-ID: <20190820023031.GC9594@infradead.org>
Date:   Mon, 19 Aug 2019 19:30:31 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     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
Subject: Re: [PATCH] btrfs: fix allocation of bitmap pages.

On Mon, Aug 19, 2019 at 07:46:00PM +0200, David Sterba wrote:
> Another thing that is lost is the slub debugging support for all
> architectures, because get_zeroed_pages lacking the red zones and sanity
> checks.
> 
> I find working with raw pages in this code a bit inconsistent with the
> rest of btrfs code, but that's rather minor compared to the above.
> 
> Summing it up, I think that the proper fix should go to copy_page
> implementation on architectures that require it or make it clear what
> are the copy_page constraints.

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 :(