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Date: Thu, 9 Nov 2023 09:27:11 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Andreas =?ISO-8859-1?Q?Gr=FCnbacher?= <andreas.gruenbacher@gmail.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>, linux-ext4
 <linux-ext4@vger.kernel.org>, gfs2@lists.linux.dev, Linux FS-devel Mailing
 List <linux-fsdevel@vger.kernel.org>, linux-xfs
 <linux-xfs@vger.kernel.org>, "Darrick J . Wong" <djwong@kernel.org>,
 linux-erofs@lists.ozlabs.org, "Theodore Ts'o" <tytso@mit.edu>, Andreas
 Dilger <adilger.kernel@dilger.ca>, Andreas Gruenbacher
 <agruenba@redhat.com>
Subject: Re: [PATCH 1/3] mm: Add folio_zero_tail() and use it in ext4
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On Thu, 9 Nov 2023 01:12:15 +0100 Andreas Gr=FCnbacher <andreas.gruenbacher=
@gmail.com> wrote:

> Andrew,
>=20
> Andrew Morton <akpm@linux-foundation.org> schrieb am Do., 9. Nov. 2023, 0=
0:06:
> > > +
> > > +     if (folio_test_highmem(folio)) {
> > > +             size_t max =3D PAGE_SIZE - offset_in_page(offset);
> > > +
> > > +             while (len > max) {
> >
> > Shouldn't this be `while (len)'?  AFAICT this code can fail to clear
> > the final page.
>=20
> not sure what you're seeing there, but this looks fine to me.

I was right!  This code does fail to handle the final page.

: static inline void folio_fill_tail(struct folio *folio, size_t offset,
: 		const char *from, size_t len)
: {
: 	char *to =3D kmap_local_folio(folio, offset);
:=20
: 	VM_BUG_ON(offset + len > folio_size(folio));
:=20
: 	if (folio_test_highmem(folio)) {
: 		size_t max =3D PAGE_SIZE - offset_in_page(offset);
:=20
: 		while (len > max) {
: 			memcpy(to, from, max);
: 			kunmap_local(to);
: 			len -=3D max;
: 			from +=3D max;
: 			offset +=3D max;
: 			max =3D PAGE_SIZE;
: 			to =3D kmap_local_folio(folio, offset);
: 		}
: 	}
:=20
: 	memcpy(to, from, len);

This code down here handles it, doh.

: 	to =3D folio_zero_tail(folio, offset, to);
: 	kunmap_local(to);
: }

Implementation seems less straightforward than it might be?  Oh well.

Has it been runtime tested?

Anyway, let's please change the function argument ordering and remember
to cc linux-mm on v2?