On Thu, 27 Jun 2024 16:16:01 -0700 Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx> 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]
>
> 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.
>
The remainder of mm-unstable actually applies OK on top of this.
I applied the below as a fixup to Vivek's "mm/gup: introduce
memfd_pin_folios() for pinning memfd folios". After this, your v1
patch reverts cleanly.
--- a/mm/gup.c~mm-gup-introduce-memfd_pin_folios-for-pinning-memfd-folios-fix
+++ a/mm/gup.c
@@ -3856,14 +3856,15 @@ long memfd_pin_folios(struct file *memfd
next_idx != folio_index(fbatch.folios[i]))
continue;
- folio = try_grab_folio(&fbatch.folios[i]->page,
- 1, FOLL_PIN);
- if (!folio) {
+ if (try_grab_folio(fbatch.folios[i],
+ 1, FOLL_PIN)) {
folio_batch_release(&fbatch);
ret = -EINVAL;
goto err;
}
+ folio = fbatch.folios[i];
+
if (nr_folios == 0)
*offset = offset_in_folio(folio, start);
_
