linux-kernel - Re: [PATCH v3 7/8] ext4: Use rbtrees to manage PAs instead of inode i_prealloc

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <542D3378-3214-4D0D-AA63-5A149E2B00EE@dilger.ca>
Date:   Thu, 16 Feb 2023 10:07:39 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Jan Kara <jack@...e.cz>
Cc:     Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        rookxu <brookxu.cn@...il.com>,
        Ritesh Harjani <ritesh.list@...il.com>
Subject: Re: [PATCH v3 7/8] ext4: Use rbtrees to manage PAs instead of inode
 i_prealloc_list

On Feb 10, 2023, at 7:37 AM, Jan Kara <jack@...e.cz> wrote:
> So I belive mballoc tries to align everything (offsets & lengths)
> to powers of two to reduce fragmentation and simplify the work for
> the buddy allocator.  If ac->ac_b_ex.fe_len is a power-of-two, the
> alignment makes sense. But once we had to resort to higher allocator
> passes and just got some random-length extent, the alignment stops
> making sense.

In addition to optimizing for the buddy allocator, the other reason that
the allocations are aligned to power-of-two offsets is to better align
with underlying RAID stripes.  Otherwise, unaligned writes will cause
parity read-modify-write updates to multiple RAID stripes.  This alignment
can also help (though to a lesser degree) with NAND flash erase blocks.

Cheers, Andreas






Download attachment "signature.asc" of type "application/pgp-signature" (874 bytes)