On Thu, Dec 08, 2022 at 02:14:42PM +0100, David Hildenbrand wrote: > On 07.12.22 21:30, Peter Xu wrote: > > Since walk_hugetlb_range() walks the pgtable, it needs the vma lock > > to make sure the pgtable page will not be freed concurrently. > > > > Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > > --- > > arch/s390/mm/gmap.c | 2 ++ > > fs/proc/task_mmu.c | 2 ++ > > include/linux/pagewalk.h | 11 ++++++++++- > > mm/hmm.c | 15 ++++++++++++++- > > mm/pagewalk.c | 2 ++ > > 5 files changed, 30 insertions(+), 2 deletions(-) > > > > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c > > index 8947451ae021..292a54c490d4 100644 > > --- a/arch/s390/mm/gmap.c > > +++ b/arch/s390/mm/gmap.c > > @@ -2643,7 +2643,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr, > > end = start + HPAGE_SIZE - 1; > > __storage_key_init_range(start, end); > > set_bit(PG_arch_1, &page->flags); > > + hugetlb_vma_unlock_read(walk->vma); > > cond_resched(); > > + hugetlb_vma_lock_read(walk->vma); > > return 0; > > } > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > > index e35a0398db63..cf3887fb2905 100644 > > --- a/fs/proc/task_mmu.c > > +++ b/fs/proc/task_mmu.c > > @@ -1613,7 +1613,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask, > > frame++; > > } > > + hugetlb_vma_unlock_read(walk->vma); > > cond_resched(); > > + hugetlb_vma_lock_read(walk->vma); > > We already hold the mmap_lock and reschedule. Even without the > cond_resched() we might happily reschedule on a preemptive kernel. So I'm > not sure if this optimization is strictly required or even helpful in > practice here. It's just low hanging fruit if we need that complexity anyway. That's also why I didn't do that for v1 (where I missed hmm special case, though..), but I think since we'll need that anyway, we'd better release the vma lock if we can easily do so. mmap_lock is just more special because it needs more work in the caller to release (e.g. vma invalidations). Otherwise I'm happy dropping that too. > > In the worst case, concurrent unsharing would have to wait. > For example, s390_enable_skey() is called at most once for a VM, for most > VMs it gets never even called. > > Or am I missing something important? Nothing important. I just don't see why we need to strictly follow the same release rule of mmap_lock here when talking about vma lock. In short - if we can drop a lock earlier before sleep, why not? I tend to just keep it as-is, but let me know if you have further thoughts or concerns. -- Peter Xu
