Re: [PATCH v2 2/2] mm/memfd_luo: keep memfd frozen until finish

[Chris Down <chris@chrisdown.name>]

Hi Pratyush,

Pratyush Yadav writes:
>+ * Restrictions
>+ * ============
>+ *
>+ * While a memfd is preserved, and after it is retrieved until the session is
>+ * finished, its size can't be changed and holes can't be punched in it.
>+ * Operations that would do so (``ftruncate()``, ``fallocate()``, writes past
>+ * the end of the file) fail.

Not a big deal, but there's one more thing that fails in this window and
isn't listed here. Adding F_SEAL_WRITE to a retrieved memfd before finish
now waits about 150ms and then fails with EBUSY. That's because
memfd_wait_for_pins() sees the reference kept from kho_restore_folio() as
an extra reference on every folio. Once the session is finished it
succeeds straight away. The same already happens while the memfd is
preserved in the old kernel, so maybe it's worth mentioning here for both?

Also, fallocate() fails with EPERM in every mode while the file is frozen.
That includes FALLOC_FL_KEEP_SIZE inside i_size, which neither changes the
size nor punches a hole. "Operations that would do so" makes it sound like
only some fallocate() calls fail.

Otherwise I've tested this across a kexec and it works well.

Once those are fixed, please feel free to add:

Reviewed-by: Chris Down <chris@chrisdown.name>
Tested-by: Chris Down <chris@chrisdown.name>

Thanks!

Chris