Re: [PATCH] iov_iter: fix advancing slot in iter_folioq_get_pages()

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-09-30 at 11:55 -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> iter_folioq_get_pages() decides to advance to the next folioq slot when
> it has reached the end of the current folio. However, it is checking
> offset, which is the beginning of the current part, instead of
> iov_offset, which is adjusted to the end of the current part, so it
> doesn't advance the slot when it's supposed to. As a result, on the next
> iteration, we'll use the same folio with an out-of-bounds offset and
> return an unrelated page.
> 
> This manifested as various crashes and other failures in 9pfs in drgn's
> VM testing setup and BPF CI.
> 
> Fixes: db0aa2e9566f ("mm: Define struct folio_queue and ITER_FOLIOQ to handle a sequence of folios")
> Link: https://lore.kernel.org/linux-fsdevel/20240923183432.1876750-1-chantr4@gmail.com/
> Tested-by: Manu Bretelle <chantr4@gmail.com>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---

Tried this on top of the following commit from net-fs-fixes branch:
e1b0d67c7ae0 ("9p: Don't revert the I/O iterator after reading")

The boot issue is gone, thank you!

Tested-by: Eduard Zingerman <eddyz87@gmail.com>

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