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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 4C9E5180008 X-Stat-Signature: ajayyxe3dxy317fzdjj687xngzrxbhah X-Rspam-User: X-HE-Tag: 1738229832-669301 X-HE-Meta: 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 yoGibI8u 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 30.01.25 01:27, Alistair Popple wrote: > On Wed, Jan 29, 2025 at 12:58:02PM +0100, David Hildenbrand wrote: >> Let's document how this function is to be used, and why the requirement >> for the folio lock might maybe be dropped in the future. > > Sorry, only just catching up on your other thread. The folio lock was to ensure > the GPU got a chance to make forward progress by mapping the page. Without it > the CPU could immediately invalidate the entry before the GPU had a chance to > retry the fault. > > Obviously performance wise having such thrashing is terrible, so should > really be avoided by userspace, but the lock at least allowed such programs > to complete. Thanks for the clarification. So it's relevant that the MMU notifier in remove_device_exclusive_entry() is sent after taking the folio lock. However, as soon as we drop the folio lock, remove_device_exclusive_entry() will become active, lock the folio and trigger the MMU notifier. So the time it is actually mapped into the device is rather > >> Signed-off-by: David Hildenbrand >> --- >> mm/memory.c | 25 +++++++++++++++++++++++++ >> 1 file changed, 25 insertions(+) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 46956994aaff..caaae8df11a9 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -718,6 +718,31 @@ struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma, >> } >> #endif >> >> +/** >> + * restore_exclusive_pte - Restore a device-exclusive entry >> + * @vma: VMA covering @address >> + * @folio: the mapped folio >> + * @page: the mapped folio page >> + * @address: the virtual address >> + * @ptep: PTE pointer into the locked page table mapping the folio page >> + * @orig_pte: PTE value at @ptep >> + * >> + * Restore a device-exclusive non-swap entry to an ordinary present PTE. >> + * >> + * The folio and the page table must be locked, and MMU notifiers must have >> + * been called to invalidate any (exclusive) device mappings. In case of >> + * fork(), MMU_NOTIFY_PROTECTION_PAGE is triggered, and in case of a page >> + * fault MMU_NOTIFY_EXCLUSIVE is triggered. >> + * >> + * Locking the folio makes sure that anybody who just converted the PTE to >> + * a device-private entry can map it into the device, before unlocking it; so >> + * the folio lock prevents concurrent conversion to device-exclusive. > > I don't quite follow this - a concurrent conversion would already fail > because the GUP in make_device_exclusive_range() would most likely cause > an unexpected reference during the migration. And if a migration entry > has already been installed for the device private PTE conversion then > make_device_exclusive_range() will skip it as a non-present entry anyway. Sorry, I meant "device-exclusive", so migration is not a concern. > > However s/device-private/device-exclusive/ makes sense - the intent was to allow > the device to map it before a call to restore_exclusive_pte() (ie. a CPU fault) > could convert it back to a normal PTE. > >> + * TODO: the folio lock does not protect against all cases of concurrent >> + * page table modifications (e.g., MADV_DONTNEED, mprotect), so device drivers >> + * must already use MMU notifiers to sync against any concurrent changes > > Right. It's expected drivers are using MMU notifiers to keep page tables in > sync, same as for hmm_range_fault(). Let me try to rephrase it given that the folio lock is purely to guarantee forward-progress, not for correctness; that's what MMU notifiers must be used for. -- Cheers, David / dhildenb