Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

[Catalin Marinas]

On Wed, Oct 20, 2021 at 08:19:40PM -1000, Linus Torvalds wrote:
> On Wed, Oct 20, 2021 at 12:44 PM Catalin Marinas
> <catalin.marinas@xxxxxxx> wrote:
> >
> > However, with MTE doing both get_user() every 16 bytes and
> > gup can get pretty expensive.
> 
> So I really think that anything that is performance-critical had
> better only do the "fault_in_write()" code path in the cold error path
> where you took a page fault.
[...]
> So I wouldn't worry too much about the performance concerns. It simply
> shouldn't be a common or hot path.
> 
> And yes, I've seen code that does that "fault_in_xyz()" before the
> critical operation that cannot take page faults, and does it
> unconditionally.
> 
> But then it isn't the "fault_in_xyz()" that should be blamed if it is
> slow, but the caller that does things the wrong way around.

Some more thinking out loud. I did some unscientific benchmarks on a
Raspberry Pi 4 with the filesystem in a RAM block device and a
"dd if=/dev/zero of=/mnt/test" writing 512MB in 1MB blocks. I changed
fault_in_readable() in linux-next to probe every 16 bytes:

- ext4 drops from around 261MB/s to 246MB/s: 5.7% penalty

- btrfs drops from around 360MB/s to 337MB/s: 6.4% penalty

For generic_perform_write() Dave Hansen attempted to move the fault-in
after the uaccess in commit 998ef75ddb57 ("fs: do not prefault
sys_write() user buffer pages"). This was reverted as it was exposing an
ext4 bug. I don't whether it was fixed but re-applying Dave's commit
avoids the performance drop.

btrfs_buffered_write() has a comment about faulting pages in before
locking them in prepare_pages(). I suspect it's a similar problem and
the fault_in() could be moved, though I can't say I understand this code
well enough.

Probing only the first byte(s) in fault_in() would be ideal, no need to
go through all filesystems and try to change the uaccess/probing order.

-- 
Catalin