Re: [PATCH 08/15] ubifs: Convert ubifs_write_begin() to use a folio

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 22, 2024 at 07:51:03PM +0800, Zhihao Cheng wrote:
> > @@ -437,13 +437,14 @@ static int ubifs_write_begin(struct file *file, struct address_space *mapping,
> >   		return -EROFS;
> >   	/* Try out the fast-path part first */
> > -	page = grab_cache_page_write_begin(mapping, index);
> > -	if (unlikely(!page))
> > -		return -ENOMEM;
> > +	folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
> > +			mapping_gfp_mask(mapping));
> > +	if (IS_ERR(folio))
> > +		return PTR_ERR(folio);
> > -	if (!PageUptodate(page)) {
> > +	if (!folio_test_uptodate(folio)) {
> >   		/* The page is not loaded from the flash */
> > -		if (!(pos & ~PAGE_MASK) && len == PAGE_SIZE) {
> > +		if (pos == folio_pos(folio) && len >= folio_size(folio)) {
> 
> Why not len == folio_size(folio)? Although 'len >= folio_size(folio)' is not
> wrong.

This is based on my experience with iomap.  Today, the caller passes in
min(length-of-write, PAGE_SIZE).  In order to be able to create large
folios in the write path, we want to change the caller of write_begin
to pass in the length of the write, so we can see a len which is larger
than the size of the folio we found.


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