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U2FsdGVkX18HmxHzmwBHd/iH8tXNzlzzGmACoB3kMUmqjbbJ8SnWihqzQfVLuY1W8pVusEnpn+XZx3EIgXqjGW5gtGP+3rb3AJNofbve4Wr1RRdvrfLhRRuQzV6Me9qGqC94j0ojXvO2R8ARfDcG+W8/cPDkxUf/jMeOSlB1+/K01vTpkjKjsvTmb1/cFNc6VAQDFtQUXIRmojk2D5t8arBuEpQaJCEXT/GdMXqt+n2NSxBW9G+AYFeUvixWVYkzNf4gN1qF94iDYHyIcwwwciuQcRfoW5WVP09me4ABIpvygVxEGBAfy1tLqhb2f6XPiXbYYJRXz4hh1xGWSOqpA6no8sXDNmiz/BIuXLjC6Wbcbq/7w0dqvw3gagoCBQaQLklY1aSBxIHFRnCRSrlb9v460wJyQf8VHcY4KLR3S50gqdvCUFZiGgBoebsiWWoxWZjrAxTZ8y2KTaTD/N41+MSG/ph5C7ii7hOuTKcFALHMjODuijT2yjATlasQIvr/pNLPmW+PQXI8PPBnvO0HhF4PSTqPKI9KdFzhb9pwkUZFEIHjHvPhONnlpHDqGYwXT9fGxRoxqH9EI1UwwAJ9//wsta4OU0fT2PQfqbuvvDiBsasoDyV0fLCBvPfHic/bTJzu84LaN2QPb2v7Z35OzflJX9ZwUyJWRpcZZOaeGP2WDQ8nmgX0H/x9RjU5DQyRzc+/5wCJTNZSig/2m55icadESLGQk8tBSSs3wzjITKHBBNNxzO9PX5ynpJro1BcLXkm5yir68MxnEh1fypETIqQjgr+itWO06L6D2blUzGRBGjvtkI4sgFv+0hjOAMBvwN4ep9S4HTGdU7PCheDJTW0xCK5/nsYgXYdoz3OjOmoka1f2bGgYQp6mjM7nOpa3kjCIa7ZmbtwrOH4vqNMmVJge6GBbjYgP6TKrZLRyRgYF9CpOAs1vl3XrHDbVKcJZc3ax8qeIoNSfARqcwpT z8WSyN6k gFZIrc3zEksj1mdiiXD/uEfT92kI1+PH2Qr7m1ihNDl0Yu+tNXmzFM1o2jnhRZW5zDFfm34x/e/UBziZfBEz7BIqQI8ejhGpR/3yPBibHNn3xhUMSx6wyAkkzZnPiCjUI3ih1S1bN2V6ptfVumGqkSwNtLbXNwg6wbwGwuBEB1UbyQYjjoNg/HGUv9qhPQnD7baQ9Ydsd2ccGd3tMpeXafSMLjKOcTpE6nRuuPnPPufMfz4oi/t1FL6fhFGaGl4lALcsseS2O9tykvwl9BHc1glFp6AdOdHHh11rZzVc1U7AQIm5zhi12UPZU6pqN/Jz5cu6NqLoqlfc+tu119LMvPVBFpGAuRIoHC9WNZTGtN31MouwJ3ZVeBPPKAXXOtXXB+gecTxV5VbDDkvF3eQ8R21T9CJNo0e5+eRwqDRF8vKjtzF8kwogA+favrLGLxb49i37OrV6WBM+6QlKPcZH0AWqHc6J6fxxIBhXjAdVcSr+aZ981pVoe75WhRMbfFU0eu5Fp//hXiogdEUsvaX968ODMGQ== 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 8/15/25 17:11, Mika Penttilä wrote: > On 8/15/25 08:23, Alistair Popple wrote: > >> On Thu, Aug 14, 2025 at 08:45:43PM +0300, Mika Penttilä wrote: >>> On 8/14/25 20:20, Jason Gunthorpe wrote: >>> >>>> On Thu, Aug 14, 2025 at 08:00:01PM +0300, Mika Penttilä wrote: >>>>> as well as hmm test module with : >>>>> >>>>> * Ignore invalidation callbacks for device private pages since >>>>> * the invalidation is handled as part of the migration process. >>>>> */ >>>>> if (range->event == MMU_NOTIFY_MIGRATE && >>>>> range->owner == dmirror->mdevice) >>>>> return true; >>>> If I recall this was about a very specific case where migration does a >>>> number of invalidations and some of the earlier ones are known to be >>>> redundant in this specific case. Redundant means it can be ignored >>>> without causing an inconsistency. >>>> >>>> Alistair would know, but I assumed this works OK because the above >>>> invalidation doesn't actually go on to free any pages but keeps them >>>> around until a later invalidation? > > Thanks Alistair for your deep insights! > >> Right, the pages don't actually get freed because a reference is taken on them >> during migrate_vma_setup(). However other device MMU's still need invalidating >> because the driver will go on to copy the page after this step. It's just >> assumed that the driver is able to be consistent with itself (ie. it will unmap/ >> invalidate it's own MMU prior to initiating the copy). > > And reference is taken as well in migrate on fault during hmm_range_fault > if migrating. > >> >> In practice I suspect what Mika is running into is that the page table >> synchronisation for migration works slightly differently for migrate_vma_*(). >> >> Instead of using mmu_interval_notifier's which have a sequence number drivers >> typically use normal mmu_notifier's and take a device specific lock to block >> page table downgrades (eg. RW -> RO). This ensures it's safe to update the >> device page tables with the PFNs/permissions collected in migrate_vma_setup() >> (or the new PFN) by blocking other threads from updating the page table. >> >> The ususal problem with this approach is that when migrate_vma_setup() calls >> the mmu_notifier it deadlocks on the device specific lock in the notifier >> callback because it already holds the lock, which it can't drop before calling >> migrate_vma_setup(). >> >> I think one of the main benefits of a series which consolidates these two >> page-table mirroring techniques into common code would also be to make the >> mirroring/invalidation logic the same for migration as hmm_range_fault(). Ie. to >> move to mmu_interval notifers with sequence numbers for migration, perhaps with >> filtering if required/safe and retries > > Yes with the migrate_vma_setup() and collecting removed, the firing of mmu notifiers > and "collecting" are integral part of the hmm_range_fault() flow, so logical to use > interval notifiers for migrate also. > > I have removed the commit with the owner games. I studied it more and seems it was added > to mitigate a bug in an early version, which led me to do wrong conclusion of the root cause > of the hang. That version had unbalanced mmu_notifier_invalidate_range_start() > after returning from hmm_range_fault() with EBUSY (after done a folio split). > With that fixed, driving the migrate on fault using the interval notifiers seems to work well, > filtering MMU_NOTIFY_MIGRATE for device for retries. > So this patch can be ignored in the series? Balbir