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

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Message-ID: <40be01d6cc61$7d23cac0$776b6040$@samsung.com>
Date:   Mon, 7 Dec 2020 15:23:33 +0900
From:   "Sungjong Seo" <sj1557.seo@...sung.com>
To:     "'Artem Labazov'" <123321artyom@...il.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".

The address range available for vmalloc() allocations is limited on 32-bit
systems. If all kernel codes that need non-contiguous memory of the size
order 1 or larger try to allocate it by only vmalloc(), the address range
for vmalloc() could be insufficient.
So, I think it would be better to give kmalloc() "one" chance.

I know that kvmalloc() only tries kmalloc() once (noretry, nowarn) and if it
fails, it immediately falls back to vmalloc(). Therefore, I think kvmalloc()
and kvfree() are the best solution for solving the problem you are facing
and
the problem I mentioned above.

Could you send me patch v3 that uses kvcalloc() and kvfree()?

> 
> --
> Artem