Re: [PATCH] block: Fix page refcounts for unaligned buffers in __bio_release_pages()

[Tony Battersby <tonyb@cybernetics.com>]

On 2/29/24 14:53, Matthew Wilcox wrote:
> On Thu, Feb 29, 2024 at 01:08:09PM -0500, Tony Battersby wrote:
>> Fix an incorrect number of pages being released for buffers that do not
>> start at the beginning of a page.
> Oh, I see what I did.  Wouldn't a simpler fix be to just set "done" to
> offset_in_page(fi.offset)?

Actually it would be:

ssize_t done = -offset_in_page(offset);

But then you have signed vs. unsigned comparison in the while(), or you
could rearrange the loop to avoid the negative, but then it gets clunky.

>
>> @@ -1152,7 +1152,7 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
>>  
>>  	bio_for_each_folio_all(fi, bio) {
>>  		struct page *page;
>> -		size_t done = 0;
>> +		size_t nr_pages;
>>  
>>  		if (mark_dirty) {
>>  			folio_lock(fi.folio);
>> @@ -1160,10 +1160,11 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
>>  			folio_unlock(fi.folio);
>>  		}
>>  		page = folio_page(fi.folio, fi.offset / PAGE_SIZE);
>> +		nr_pages = (fi.offset + fi.length - 1) / PAGE_SIZE -
>> +			   fi.offset / PAGE_SIZE + 1;
>>  		do {
>>  			bio_release_page(bio, page++);
>> -			done += PAGE_SIZE;
>> -		} while (done < fi.length);
>> +		} while (--nr_pages != 0);
>>  	}
>>  }
>>  EXPORT_SYMBOL_GPL(__bio_release_pages);
> The long-term path here, I think, is to replace this bio_release_page()
> with a bio_release_folio(folio, offset, length) which calls into
> a new unpin_user_folio(folio, nr) which calls gup_put_folio().

I developed the patch with the 6.1 stable series, which just has:

nr_pages = (fi.offset + fi.length - 1) / PAGE_SIZE -
	   fi.offset / PAGE_SIZE + 1;
folio_put_refs(fi.folio, nr_pages);

Which is another reason that I went for the direct nr_pages
calculation.  Would you still prefer the negative offset_in_page() approach?

Tony