Re: [PATCH v2 2/3] mm: Allow architectures to request 'old' entries when prefaulting

On Mon, Jan 11, 2021 at 05:25:33PM +0300, Kirill A. Shutemov wrote:

On Fri, Jan 08, 2021 at 05:15:16PM +0000, Will Deacon wrote:

quoted

diff --git a/mm/filemap.c b/mm/filemap.c
index c1f2dc89b8a7..0fb9d1714797 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c

@@ -3051,14 +3051,18 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		if (!pte_none(*vmf->pte))
 			goto unlock;
 
+		/* We're about to handle the fault */
+		if (vmf->address == address) {
+			vmf->flags &= ~FAULT_FLAG_PREFAULT;
+			ret = VM_FAULT_NOPAGE;
+		} else {
+			vmf->flags |= FAULT_FLAG_PREFAULT;
+		}
+

Do we need to restore the oririnal status of the bit once we are done?

I can certainly add that, although it doesn't look like we do that for
vmf->pte, so it's hard to tell what the rules are here. It certainly feels
odd to restore some fields but not others, as it looks like vmf->address
will be out-of-whack with vmf->pte when filemap_map_pages() returns. Am I
missing something?

Will

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