Re: [v2 PATCH] mm: gup: do not call try_grab_folio() in slow path

[Peter Xu <peterx@redhat.com>]

Yang,

On Thu, Jun 27, 2024 at 03:14:13PM -0700, Yang Shi wrote:
> The try_grab_folio() is supposed to be used in fast path and it elevates
> folio refcount by using add ref unless zero.  We are guaranteed to have
> at least one stable reference in slow path, so the simple atomic add
> could be used.  The performance difference should be trivial, but the
> misuse may be confusing and misleading.

This first paragraph is IMHO misleading itself..

I think we should mention upfront the important bit, on the user impact.

Here IMO the user impact should be: Linux may fail longterm pin in some
releavnt paths when applied over CMA reserved blocks.  And if to extend a
bit, that include not only slow-gup but also the new memfd pinning, because
both of them used try_grab_folio() which used to be only for fast-gup.

It's great this patch renamed try_grab_folio() to try_grab_folio_fast(), I
think that definitely helps on reducing the abuse in the future.  However
then with that the subject becomes misleading, because it says "do not call
try_grab_folio()" however after this patch we keep using it.

Maybe rename the subject to "mm: Fix longterm pin on slow gup and memfd pin
regress"?

> 
> In another thread [1] a kernel warning was reported when pinning folio
> in CMA memory when launching SEV virtual machine.  The splat looks like:
> 
> [  464.325306] WARNING: CPU: 13 PID: 6734 at mm/gup.c:1313 __get_user_pages+0x423/0x520
> [  464.325464] CPU: 13 PID: 6734 Comm: qemu-kvm Kdump: loaded Not tainted 6.6.33+ #6
> [  464.325477] RIP: 0010:__get_user_pages+0x423/0x520
> [  464.325515] Call Trace:
> [  464.325520]  <TASK>
> [  464.325523]  ? __get_user_pages+0x423/0x520
> [  464.325528]  ? __warn+0x81/0x130
> [  464.325536]  ? __get_user_pages+0x423/0x520
> [  464.325541]  ? report_bug+0x171/0x1a0
> [  464.325549]  ? handle_bug+0x3c/0x70
> [  464.325554]  ? exc_invalid_op+0x17/0x70
> [  464.325558]  ? asm_exc_invalid_op+0x1a/0x20
> [  464.325567]  ? __get_user_pages+0x423/0x520
> [  464.325575]  __gup_longterm_locked+0x212/0x7a0
> [  464.325583]  internal_get_user_pages_fast+0xfb/0x190
> [  464.325590]  pin_user_pages_fast+0x47/0x60
> [  464.325598]  sev_pin_memory+0xca/0x170 [kvm_amd]
> [  464.325616]  sev_mem_enc_register_region+0x81/0x130 [kvm_amd]
> 
> Per the analysis done by yangge, when starting the SEV virtual machine,
> it will call pin_user_pages_fast(..., FOLL_LONGTERM, ...) to pin the
> memory.  But the page is in CMA area, so fast GUP will fail then
> fallback to the slow path due to the longterm pinnalbe check in
> try_grab_folio().
> The slow path will try to pin the pages then migrate them out of CMA
> area.  But the slow path also uses try_grab_folio() to pin the page,
> it will also fail due to the same check then the above warning
> is triggered.
> 
> [1] https://lore.kernel.org/linux-mm/1719478388-31917-1-git-send-email-yangge1116@126.com/
> 
> Fixes: 57edfcfd3419 ("mm/gup: accelerate thp gup even for "pages != NULL"")
> Cc: <stable@vger.kernel.org> [6.6+]
> Reported-by: yangge <yangge1116@126.com>
> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>

The patch itself looks mostly ok to me.

There's still some "cleanup" part mangled together, e.g., the real meat
should be avoiding the folio_is_longterm_pinnable() check in relevant
paths.  The rest (e.g. switch slow-gup / memfd pin to use folio_ref_add()
not try_get_folio(), and renames) could be good cleanups.

So a smaller fix might be doable, but again I don't have a strong opinion
here.

Thanks,

-- 
Peter Xu