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owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 02, 2025 at 10:48:47AM +0800, Yafang Shao wrote: > > > > > > > > > > > > > > > > > However, when you switch the THP mode to "never", tasks that still > > > > > have MMF_VM_HUGEPAGE remain on the khugepaged scan list. This isn’t an > > > > > issue under the current global mode because khugepaged doesn’t run > > > > > when THP is set to "never". > > > > > > > > > > The problem arises when we move from a global mode to a per-task mode. > > > > > In that case, khugepaged may end up doing unnecessary work. For > > > > > example, if the THP mode is "always", but some tasks are not allowed > > > > > to allocate THP while still having MMF_VM_HUGEPAGE set, khugepaged > > > > > will continue scanning them unnecessarily. > > > > > > > > But this can change right? > > > > > > > > I really don't like the idea _at all_ of overriding this hook to do things > > > > other than what it says it does. > > > > > > > > It's 'set which order to use' except when it's this case then it's 'will we > > > > do any work'. > > > > > > > > This should be a separate callback or we should drop this and live with the > > > > possible additional work. > > > > > > Perhaps we could reuse the MMF_DISABLE_THP flag by introducing a new > > > BPF helper to set it when we want to disable THP for a specific task. > > > > Interesting, yeah perhaps that could work, as long as we're in a sensible > > context to be able to toggle this bit. > > Right, we can't set the mm->flags arbitrarily. > Perhaps we should add a generic BPF hook in dup_mmap(). > Yeah perhaps that could be a way forward :) > diff --git a/mm/mmap.c b/mm/mmap.c > index 7a057e0e8da9..1b60bdb08de1 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -1843,6 +1843,8 @@ __latent_entropy int dup_mmap(struct mm_struct > *mm, struct mm_struct *oldmm) > loop_out: > vma_iter_free(&vmi); > if (!retval) { > + /* Allow a BPF program to modify the new mm_struct in fork. */ > + bpf_hook_mm_fork(mm, oldmm); > mt_set_in_rcu(vmi.mas.tree); > ksm_fork(mm, oldmm); > khugepaged_fork(mm, oldmm); > > This provides a mechanism for BPF programs to configure the new > mm_struct on demand, acting as a modern, flexible replacement for > prctl() ;-) Hahaha that's obviously very appealing to me :))) > > > > > > > > > Separately from this patchset, I realized we can optimize khugepaged > > > handling for the MMF_DISABLE_THP case with the following changes: > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > index 15203ea7d007..e9964edcee29 100644 > > > --- a/mm/khugepaged.c > > > +++ b/mm/khugepaged.c > > > @@ -402,6 +402,11 @@ void __init khugepaged_destroy(void) > > > kmem_cache_destroy(mm_slot_cache); > > > } > > > > > > +static inline int hpage_collapse_test_disable(struct mm_struct *mm) > > > +{ > > > + return test_bit(MMF_DISABLE_THP, &mm->flags); > > > +} > > > + > > > static inline int hpage_collapse_test_exit(struct mm_struct *mm) > > > { > > > return atomic_read(&mm->mm_users) == 0; > > > @@ -1448,6 +1453,11 @@ static void collect_mm_slot(struct > > > khugepaged_mm_slot *mm_slot) > > > /* khugepaged_mm_lock actually not necessary for the below */ > > > mm_slot_free(mm_slot_cache, mm_slot); > > > mmdrop(mm); > > > + } else if (hpage_collapse_test_disable(mm)) { > > > + hash_del(&slot->hash); > > > + list_del(&slot->mm_node); > > > + mm_flags_clear(MMF_VM_HUGEPAGE, mm); > > > + mm_slot_free(mm_slot_cache, mm_slot); > > > } > > > } > > > > > > Specifically, if MMF_DISABLE_THP is set, we should remove it from > > > mm_slot to prevent unnecessary khugepaged processing. > > > > Ohhh interesting, perhaps send as separate patch? > > sure, I will send it separately. Thanks! > > -- > Regards > Yafang And overall - cheers for being an ABSOLUTE DELIGHT on review :) it's much appreciated. I shall buy you a beer (or whatever is your preferred beverage) at the next conference we are both at :) Cheers, Lorenzo