linux-kernel - Re: [PATCH v2] vfs: shave work on failed file open

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Message-ID: <ZRdOkpXUva8UHfEJ@casper.infradead.org>
Date:   Fri, 29 Sep 2023 23:24:18 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] vfs: shave work on failed file open

On Fri, Sep 29, 2023 at 11:23:04PM +0200, Mateusz Guzik wrote:
> Extending struct file is not ideal by any means, but the good news is that:
> 1. there is a 4 byte hole in there, if one is fine with an int-sized counter
> 2. if one insists on 8 bytes, the struct is 232 bytes on my kernel
> (debian). still some room up to 256, so it may be tolerable?

256 isn't quite the magic number for slabs ... at 256 bytes, we'd get 16
per 4kB page, but at 232 bytes we get 17 objects per 4kB page (or 35 per
8kB pair of pages).

That said, I thik a 32-bit counter is almost certainly sufficient.