On Tue, Oct 11, 2016 at 05:47:50PM +0200, Jan Kara wrote: > On Thu 15-09-16 14:54:54, Kirill A. Shutemov wrote: > > For filesystems that wants to be write-notified (has mkwrite), we will > > encount write-protection faults for huge PMDs in shared mappings. > > > > The easiest way to handle them is to clear the PMD and let it refault as > > wriable. > > > > Signed-off-by: Kirill A. Shutemov <[email protected]> > > --- > > mm/memory.c | 11 ++++++++++- > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 83be99d9d8a1..aad8d5c6311f 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -3451,8 +3451,17 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t > > orig_pmd) > > return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd, > > fe->flags); > > > > + if (fe->vma->vm_flags & VM_SHARED) { > > + /* Clear PMD */ > > + zap_page_range_single(fe->vma, fe->address, > > + HPAGE_PMD_SIZE, NULL); > > + VM_BUG_ON(!pmd_none(*fe->pmd)); > > + > > + /* Refault to establish writable PMD */ > > + return 0; > > + } > > + > > Since we want to write-protect the page table entry on each page writeback > and write-enable then on the next write, this is relatively expensive. > Would it be that complicated to handle this fully in ->pmd_fault handler > like we do for DAX? > > Maybe it doesn't have to be done now but longer term I guess it might make > sense.
Right. This approach is just simplier to implement. We can rework it if it will show up on traces. > Otherwise the patch looks good so feel free to add: > > Reviewed-by: Jan Kara <[email protected]> Thanks! -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
