Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass

On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote:

The copy_mount_options() function takes a user pointer argument but not
a size. It tries to read up to a PAGE_SIZE. However, copy_from_user() is
not guaranteed to return all the accessible bytes if, for example, the
access crosses a page boundary and gets a fault on the second page. To
work around this, the current copy_mount_options() implementations
performs to copy_from_user() passes, first to the end of the current
page and the second to what's left in the subsequent page.

Some architectures like arm64 can guarantee an exact copy_from_user()
depending on the size (since the arch function performs some alignment
on the source register). Introduce an arch_has_exact_copy_from_user()
function and allow copy_mount_options() to perform the user access in a
single pass.

While this function is not on a critical path, the single-pass behaviour
is required for arm64 MTE (memory tagging) support where a uaccess can
trigger intra-page faults (tag not matching). With the current
implementation, if this happens during the first page, the function will
return -EFAULT.

Do you know how much extra overhead we'd incur if we read at must one
tag granule at a time, instead of PAGE_SIZE?

I'm guessing that in practice strcpy_from_user() type operations copy
much less than a page most of the time, so what we lose in uaccess
overheads we _might_ regain in less redundant copying.

Would need behchmarking though.

[...]

Cheers
---Dave

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