Re: [PATCH 3/5] mm: make buffered writes work with RWF_UNCACHED

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 10, 2019 at 09:24:52AM -0700, Jens Axboe wrote:
> +/*
> + * Start writeback on the pages in pgs[], and then try and remove those pages
> + * from the page cached. Used with RWF_UNCACHED.
> + */
> +void write_drop_cached_pages(struct page **pgs, struct address_space *mapping,
> +			     unsigned *nr)

It would seem more natural to use a pagevec instead of pgs/nr.

> +{
> +	loff_t start, end;
> +	int i;
> +
> +	end = 0;
> +	start = LLONG_MAX;
> +	for (i = 0; i < *nr; i++) {
> +		struct page *page = pgs[i];
> +		loff_t off;
> +
> +		off = (loff_t) page_to_index(page) << PAGE_SHIFT;

Isn't that page_offset()?

> +	__filemap_fdatawrite_range(mapping, start, end, WB_SYNC_NONE);
> +
> +	for (i = 0; i < *nr; i++) {
> +		struct page *page = pgs[i];
> +
> +		lock_page(page);
> +		if (page->mapping == mapping) {

So you're protecting against the page being freed and reallocated to a
different file, but not against the page being freed and reallocated to
a location in the same file which is outside (start, end)?