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2025 at 08:09:58PM +0800, Kefeng Wang wrote: > > > On 2025/11/4 17:01, Lorenzo Stoakes wrote: > > On Tue, Nov 04, 2025 at 04:34:35PM +0800, Kefeng Wang wrote: > > > > +static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavior) > > > > { > > > > + int behavior = madv_behavior->behavior; > > > > + > > > > if (is_memory_failure(behavior)) > > > > - return 0; > > > > + return MADVISE_NO_LOCK; > > > > - if (madvise_need_mmap_write(behavior)) { > > > > + switch (behavior) { > > > > + case MADV_REMOVE: > > > > + case MADV_WILLNEED: > > > > + case MADV_COLD: > > > > + case MADV_PAGEOUT: > > > > + case MADV_FREE: > > > > + case MADV_POPULATE_READ: > > > > + case MADV_POPULATE_WRITE: > > > > + case MADV_COLLAPSE: > > > > + case MADV_GUARD_INSTALL: > > > > + case MADV_GUARD_REMOVE: > > > > + return MADVISE_MMAP_READ_LOCK; > > > > + case MADV_DONTNEED: > > > > + case MADV_DONTNEED_LOCKED: > > > > + return MADVISE_VMA_READ_LOCK; > > > > > > I have a question, we will try per-vma lock for dontneed, > > > but there is a mmap_assert_locked() during madvise_dontneed_free(), > > > > Hmm, this is only in the THP PUD huge case, and MADV_FREE is only valid for > > anonymous memory, and I think only DAX can have some weird THP PUD case. > > > > So I don't think we can hit this. > > Yes, we don't support pud THP for anonymous pages. Right, so we can't hit this. > > > > > In any event, I think this mmap_assert_locked() is mistaken, as we should > > only need a VMA lock here. > > > > So we could replace with a: > > > > if (!rwsem_is_locked(&tlb->mm->mmap_lock)) > > vma_assert_locked(vma); > > > > ? > > > > The pmd dax/anon split don't have assert, for PUD dax, we maybe remove this > assert? Well, we probably do want to assert that we hold a lock. > > > > > > > > > > madvise_dontneed_free > > > madvise_dontneed_single_vma > > > zap_page_range_single_batched > > > unmap_single_vma > > > unmap_page_range > > > zap_pud_range > > > mmap_assert_locked > > > > > > We could fix it by passing the lock_mode into zap_detial and then check > > > the right lock here, but I'm not sure whether it is safe to zap page > > > only with vma lock? > > > > It's fine to zap with the VMA lock. You need only hold the VMA stable which > > a VMA lock achieves. > > > > See https://docs.kernel.org/mm/process_addrs.html > > Thanks, I will learn it. Hopefully useful, I made it to remind myself of these things as they're very fiddly + otherwise I find myself constantly forgetting these details :) > > > > > > > > > And another about 4f8ba33bbdfc （"mm: madvise: use per_vma lock > > > for MADV_FREE"）, it called walk_page_range_vma() in > > > madvise_free_single_vma(), but from link[1] and 5631da56c9a8 > > > ("fs/proc/task_mmu: read proc/pid/maps under per-vma lock"), it saids > > > > > > "Note that similar approach would not work for /proc/pid/smaps > > > reading as it also walks the page table and that's not RCU-safe" > > > > > > We could use walk_page_range_vma() instead of walk_page_range() in > > > smap_gather_stats(), and same question, why 4f8ba33bbdfc(for MADV_FREEE) > > > is safe but not for show_numa_map()/show_smap()? > > > > We only use walk_page_range() there in case 4 listed in show_smaps_rollup() > > where the mmap lock is dropped on contention. > > Sorry, I mean the walk_page_range() in smap_gather_stats() called by > show_smap() from /proc/pid/smaps, not the walk_page_range() in > show_smaps_rollup() from /proc/pid/smaps_rollup. show_smaps() -> smap_gather_stats(..., start = 0) -> walk_page_vma() Because: if (!start) walk_page_vma(vma, ops, mss); The only case where start is non-zero is show_smaps_rollup() case 4. So we are already using walk_page_vma() here right? I may be missing something here :) > > > > > > > > > > Thanks. > > > > > > [1] https://lkml.kernel.org/r/20250719182854.3166724-1-surenb@google.com > > > > AFAICT That's referring to a previous approach that tried to walk > > /proc/$pid/swaps under RCU _alone_ without VMA locks. This is not safe as > > page tables can be yanked from under you not under RCU. > > But for now it tries per-vma lock or fallback to mmap lock, not lockless, so > do you mean we could try per-vma lock for /proc/pid/numa_maps or > /proc/pid/smaps ? Probably we could, but I'm not sure if it'd be really worth it, since traversing page tables is a very heavy operation and so optimising it against contention like this seems probably not all that worth it? Suren maybe could comment on this. Cheers, Lorenzo