Re: [PATCH V2] mm/gup: Fix longterm pin on slow gup regression

[Andrew Morton]

On Fri, 28 Jun 2024 14:01:58 +0800 yangge1116@xxxxxxx wrote:

> From: yangge <yangge1116@xxxxxxx>
> 
> If a large number of CMA memory are configured in system (for
> example, the CMA memory accounts for 50% of the system memory),
> starting a SEV virtual machine will fail. During starting the SEV
> virtual machine, it will call pin_user_pages_fast(..., FOLL_LONGTERM,
> ...) to pin memory. Normally if a page is present and in CMA area,
> pin_user_pages_fast() will first call __get_user_pages_locked() to
> pin the page in CMA area, and then call
> check_and_migrate_movable_pages() to migrate the page from CMA area
> to non-CMA area. But the current code calling __get_user_pages_locked()
> will fail, because it call try_grab_folio() to pin page in gup slow
> path.
> 
> The commit 57edfcfd3419 ("mm/gup: accelerate thp gup even for "pages
> != NULL"") uses try_grab_folio() in gup slow path, which seems to be
> problematic because try_grap_folio() will check if the page can be
> longterm pinned. This check may fail and cause __get_user_pages_lock()
> to fail. However, these checks are not required in gup slow path,
> seems we can use try_grab_page() instead of try_grab_folio(). In
> addition, in the current code, try_grab_page() can only add 1 to the
> page's refcount. We extend this function so that the page's refcount
> can be increased according to the parameters passed in.
> 
> The following log reveals it:
> 
> [  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]
> 

Well, we also have Yang Shi's patch
(https://lkml.kernel.org/r/20240627231601.1713119-1-yang@xxxxxxxxxxxxxxxxxxxxxx)
which takes a significantly different approach.  Which way should we
go?