linux-kernel - Re: [PATCH v3] btrfs: fix alginment of VMA for memory mapped files on THP

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Message-ID: <20220803134033.GG13489@twin.jikos.cz>
Date:   Wed, 3 Aug 2022 15:40:33 +0200
From:   David Sterba <dsterba@...e.cz>
To:     alexlzhu@...com
Cc:     kernel-team@...com, linux-mm@...ck.org, clm@...com,
        josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] btrfs: fix alginment of VMA for memory mapped files
 on THP

On Tue, Aug 02, 2022 at 01:32:46PM -0700, alexlzhu@...com wrote:
> From: Alexander Zhu <alexlzhu@...com>
> 
> With CONFIG_READ_ONLY_THP_FOR_FS, the Linux kernel supports using THPs for
> read-only mmapped files, such as shared libraries. However, the kernel
> makes no attempt to actually align those mappings on 2MB boundaries,
> which makes it impossible to use those THPs most of the time. This issue
> applies to general file mapping THP as well as existing setups using
> CONFIG_READ_ONLY_THP_FOR_FS. This is easily fixed by using
> thp_get_unmapped_area for the unmapped_area function in btrfs, which
> is what ext2, ext4, fuse, and xfs all use.
> 
> Initially btrfs had been left out in Commit 8c07fc452ac0 ("btrfs: fix
> alginment of VMA for memory mapped files on THP") as btrfs does not support
> DAX. However, Commit 1854bc6e2420 ("mm/readahead: Align file mappings
> for non-DAX") removed the DAX requirement. We should now be able to call
> thp_get_unmapped_area() for btrfs.
> 
> The problem can be seen in /proc/PID/smaps where THPeligible is set to 0
> on mappings to eligible shared object files as shown below.
> 
> Before this patch:
> 
> 7fc6a7e18000-7fc6a80cc000 r-xp 00000000 00:1e 199856
> /usr/lib64/libcrypto.so.1.1.1k
> Size:               2768 kB
> THPeligible:    0
> VmFlags: rd ex mr mw me
> 
> With this patch the library is mapped at a 2MB aligned address:
> 
> fbdfe200000-7fbdfe4b4000 r-xp 00000000 00:1e 199856
> /usr/lib64/libcrypto.so.1.1.1k
> Size:               2768 kB
> THPeligible:    1
> VmFlags: rd ex mr mw me
> 
> This fixes the alignment of VMAs for any mmap of a file that has the
> rd and ex permissions and size >= 2MB. The VMA alignment and
> THPeligible field for anonymous memory is handled separately and
> is thus not effected by this change.
> 
> Signed-off-by: Alexander Zhu <alexlzhu@...com>

Added to misc-next, thanks.