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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 27 Jun 2024 15:14:13 -0700 Yang Shi <yang@os.amperecomputing.com> 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.
> 
> 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]
> 
> ...
>
> Fixes: 57edfcfd3419 ("mm/gup: accelerate thp gup even for "pages != NULL"")
> Cc: <stable@vger.kernel.org> [6.6+]

So we want something against Linus mainline for backporting ease.

>    3. Rebased onto the latest mm-unstable

mm-unstable is quite different - memfd_pin_folios() doesn't exist in
mainline!

So can you please prepare the fix against current -linus?  I'll hang
onto this patch to guide myself when I redo Vivek's "mm/gup: Introduce
memfd_pin_folios() for pinning memfd folios" series on top.