linux-kernel - Re: [PATCH] exfat: Avoid allocating upcase table using kcalloc()

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Date:   Fri, 4 Dec 2020 15:57:33 +0300
From:   Artem Labazov <123321artyom@...il.com>
To:     Sungjong Seo <sj1557.seo@...sung.com>
Cc:     stable@...r.kernel.org, 'Namjae Jeon' <namjae.jeon@...sung.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exfat: Avoid allocating upcase table using kcalloc()

> I have not yet received a report of the same issue.
> But I agree that this problem is likely to occur even if it is low
> probability.

Perhaps I should clarify my setup a little bit more.
The issue can be reliably reproduced on my laptop. It has 8 GBs of RAM
(pretty common amount nowadays) and runs an unmodified Fedora 32 kernel.
Also, I use zswap, which seems to be contributing to fragmentation as well.

> I think it would be more appropriate to use kvcalloc and kvfree instead.

I do not think this is really needed.
Upcase table allocation is relatively large (32 pages of 4KB size) and
happens only once, when the drive is being mounted. Also, exfat driver
does not rely on the fact that the table is physically contiguous.
That said, vmalloc/vfree seems to be the best option, according to kernel's
"Memory Allocation Guide".

--
Artem