linux-kernel - Re: [PATCH] fuse: enable larger read buffers for readdir [v2].

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Message-ID: <c4d98da8-1931-4165-9212-c502c71d4bbd@uls.co.za>
Date:   Fri, 28 Jul 2023 07:05:06 +0200
From:   Jaco Kroon <jaco@....co.za>
To:     Bernd Schubert <bernd.schubert@...tmail.fm>,
        Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Antonio SJ Musumeci <trapexit@...wn.link>
Subject: Re: [PATCH] fuse: enable larger read buffers for readdir [v2].

Hi,
>>>          plus = fuse_use_readdirplus(inode, ctx);
>>>          ap->args.out_pages = true;
>>> -       ap->num_pages = 1;
>>> +       ap->num_pages = READDIR_PAGES;
>>
>> No.  This is the array lenght, which is 1.  This is the hack I guess,
>> which makes the above trick work.
>
> Hmm, ap->num_pages / ap->pages[] is used in fuse_copy_pages, but so is 
> ap->descs[] - shouldn't the patch caused an out-of-bound access?
> Out of interest, would you mind to explain how the hack worked?

Apparently it shouldn't ... my understanding of how pages* worked was 
all wrong.

I'm guessing since all the data fits in the first page (ap->pages[0] in 
other words, of length/size desc.length) that the other pages are never 
accessed.  Looking at fuse_copy_pages this does indeed seem to be the 
case.  So I ended up just being really, really lucky here.

Kind regards,
Jaco