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?
