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From:   Andrei Vagin <avagin@gmail.com>
Date:   Mon, 24 Jan 2022 19:21:44 -0800
Message-ID: <CANaxB-x83ZUSw493feutgF4_sBD7Uj_p0VwNxpmaq1u7eMsq-w@mail.gmail.com>
Subject: Re: [PATCH] fs/pipe: use kvcalloc to allocate a pipe_buffer array
To:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian.brauner@ubuntu.com>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>
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On Tue, Jan 4, 2022 at 9:11 AM Andrei Vagin <avagin@gmail.com> wrote:
>
> Right now, kcalloc is used to allocate a pipe_buffer array.  The size of
> the pipe_buffer struct is 40 bytes. kcalloc allows allocating reliably
> chunks with sizes less or equal to PAGE_ALLOC_COSTLY_ORDER (3). It means
> that the maximum pipe size is 3.2MB in this case.
>
> In CRIU, we use pipes to dump processes memory. CRIU freezes a target
> process, injects a parasite code into it and then this code splices
> memory into pipes. If a maximum pipe size is small, we need to
> do many iterations or create many pipes.
>
> kvcalloc attempt to allocate physically contiguous memory, but upon
> failure, fall back to non-contiguous (vmalloc) allocation and so it
> isn't limited by PAGE_ALLOC_COSTLY_ORDER.
>
> The maximum pipe size for non-root users is limited by
> the /proc/sys/fs/pipe-max-size sysctl that is 1MB by default, so only
> the root user will be able to trigger vmalloc allocations.
>
> Cc: Dmitry Safonov <0x7f454c46@gmail.com>
> Signed-off-by: Andrei Vagin <avagin@gmail.com>

Alexander and Andrew,

Do you have any objections? Could you merge this patch?

Dmitry's comments have been addressed in the separate patch:
https://lkml.org/lkml/2022/1/6/463

BTW: I found that fuse already uses kvmalloc to allocate pipe buffers
(e. g. fuse_dev_splice_read).

Thanks,
Andrei