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

Jens Axboe
2019-12-10T17:04:25Z

On 12/10/19 9:55 AM, Matthew Wilcox wrote:
> 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.

I did look into that, but they are intertwined with LRU etc. I
deliberately avoided the LRU on the read side, as it adds noticeable
overhead and gains us nothing since the pages will be dropped agian.

>> +{
>> + 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()?

I guess it is! I'll make that change.

>> + __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)?

I guess so, we can add that too, probably just check if the index is
still the same. More of a behavioral thing, shouldn't be any
correctness issues there.

--
Jens Axboe