[PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

[Hugh Dickins]

On Sat, 26 Dec 2020, Kirill A. Shutemov wrote:
> On Sat, Dec 26, 2020 at 09:57:13AM -0800, Linus Torvalds wrote:
> > Because not only does that get rid of the "if (page)" test, I think it
> > would make things a bit clearer. When I read the patch first, the
> > initial "next_page()" call confused me.
> 
> Agreed. Here we go:
> 
> From d12dea4abe94dbc24b7945329b191ad7d29e213a Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> Date: Sat, 19 Dec 2020 15:19:23 +0300
> Subject: [PATCH] mm: Cleanup faultaround and finish_fault() codepaths
> 
> alloc_set_pte() has two users with different requirements: in the
> faultaround code, it called from an atomic context and PTE page table
> has to be preallocated. finish_fault() can sleep and allocate page table
> as needed.
> 
> PTL locking rules are also strange, hard to follow and overkill for
> finish_fault().
> 
> Let's untangle the mess. alloc_set_pte() has gone now. All locking is
> explicit.
> 
> The price is some code duplication to handle huge pages in faultaround
> path, but it should be fine, having overall improvement in readability.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>

Hold on. I guess this one will suffer from the same bug as the previous.
I was about to report back, after satisfactory overnight testing of that
version - provided that one big little bug is fixed:

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2919,7 +2919,7 @@ static bool filemap_map_pmd(struct vm_fa
 
 	if (pmd_none(*vmf->pmd) &&
 	    PageTransHuge(page) &&
-	    do_set_pmd(vmf, page)) {
+	    do_set_pmd(vmf, page) == 0) {
 		unlock_page(page);
 		return true;
 	}

(Yes, you can write that as !do_set_pmd(vmf, page), and maybe I'm odd,
but even though it's very common, I have a personal aversion to using
"!' on a positive-sounding function that returns 0 for success.)

I'll give the new patch a try now, but with that fix added in. Without it,
I got "Bad page" on compound_mapcount on file THP pages - but I run with
a BUG() inside of bad_page() so I cannot miss them: I did not look to see
what the eventual crash or page leak would look like without that.

Hugh