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

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Message-ID: <6f99b73c-db8f-8135-b827-0a135734d7da@suse.cz>
Date:   Tue, 20 Aug 2019 13:06:25 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     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
Subject: Re: [PATCH] btrfs: fix allocation of bitmap pages.

On 8/20/19 4:30 AM, Christoph Hellwig wrote:
> 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 :(

Meh, I should finally get back to https://lwn.net/Articles/787740/ right