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Fri, 5 Dec 2025 06:22:28 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: AZXKNX32rXGF Date: Fri, 05 Dec 2025 12:22:07 +0100 From: "Arnd Bergmann" To: "Andreas Dilger" , "Jan Kara" , "Arnd Bergmann" Cc: "Theodore Ts'o" , "Darrick J. Wong" , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org Message-Id: In-Reply-To: References: <20251204101914.1037148-1-arnd@kernel.org> <3ueamfhbmtwmclmtm77msvsuylgxabt3zqkrtvxqtajqhupfdd@vy7bw3e3wiwn> Subject: Re: [PATCH] ext4: fix ext4_tune_sb_params padding Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Fri, Dec 5, 2025, at 11:17, Andreas Dilger wrote: >> On Dec 4, 2025, at 3:31=E2=80=AFAM, Jan Kara wrote: >> On Thu 04-12-25 11:19:10, Arnd Bergmann wrote: > > While this change isn't _wrong_ per-se, it does seem very strange to h= ave > a 68-byte padding at the end of the struct. You have to check the num= ber > of __u32 fields closely to see this,=20 I had the same thought but decided against that because it would be an ABI break on all architectures. The version I posted only changes the structure size on x86-32, csky, m68k and microblaze, as far as I can tell. > and I wonder if this will perpetuate > errors in the future (e.g. adding a __u64 field after mount_opts[64]). Indeed, I can see how that could become worse. > IMHO, it would be more clear to either add an explicit "__u32 pad_3;" > field after mount_opts[64], or alternately declare mount_opts[68] so it > will consume those bytes and leave the remaining fields properly align= ed. > It isn't critical if the user tools use the last 4 bytes of mount_opts= [] > or not, so they could be changed independently at some later time. > > Either will ensure that new fields added in place of pad[64] will be > properly aligned in the future. Changing mount_opts[] to 68 bytes sounds fine to me, I'll send an updated patch for that. I've kept the Ack from Jan, please shout if I should drop that instead. Arnd