linux-ext4 - Re: [PATCH 1/3] mm: Add folio_zero

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Date:   Thu, 9 Nov 2023 09:27:11 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>, gfs2@...ts.linux.dev,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        linux-erofs@...ts.ozlabs.org, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andreas Gruenbacher <agruenba@...hat.com>
Subject: Re: [PATCH 1/3] mm: Add folio_zero_tail() and use it in ext4

On Thu, 9 Nov 2023 01:12:15 +0100 Andreas Grünbacher <andreas.gruenbacher@...il.com> wrote:

> Andrew,
> 
> Andrew Morton <akpm@...ux-foundation.org> schrieb am Do., 9. Nov. 2023, 00:06:
> > > +
> > > +     if (folio_test_highmem(folio)) {
> > > +             size_t max = 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.
> 
> 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 = kmap_local_folio(folio, offset);
: 
: 	VM_BUG_ON(offset + len > folio_size(folio));
: 
: 	if (folio_test_highmem(folio)) {
: 		size_t max = PAGE_SIZE - offset_in_page(offset);
: 
: 		while (len > max) {
: 			memcpy(to, from, max);
: 			kunmap_local(to);
: 			len -= max;
: 			from += max;
: 			offset += max;
: 			max = PAGE_SIZE;
: 			to = kmap_local_folio(folio, offset);
: 		}
: 	}
: 
: 	memcpy(to, from, len);

This code down here handles it, doh.

: 	to = 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?