On Wed, Jan 08, 2025 at 05:32:54PM +0000, Matthew Wilcox wrote: > On Wed, Jan 08, 2025 at 04:18:42PM +0000, Lorenzo Stoakes wrote: > > @@ -280,7 +269,10 @@ static void fb_deferred_io_work(struct work_struct *work) > > struct folio *folio = page_folio(pageref->page); > > > > folio_lock(folio); > > - folio_mkclean(folio); > > + rmap_wrprotect_file_page(fbdefio->mapping, > > + pageref->offset >> PAGE_SHIFT, > > + compound_nr(pageref->page), > > + page_to_pfn(pageref->page)); > > folio_unlock(folio); > > Why do we need to lock the folio? (since this isn't necessarily a > folio) Also, do we need compound_nr() here? I _think_ for defio, > the number of pages allocated per object are fixed, so this should be > an fbdefio->nr_pages field? I'm trying to keep the code as similar as possible to the way it was before, even if there are questionable parts. There is a comment about some timing issue around the locks and so there appears to be an assumption about that. As to compound_nr(), we're not write protecting everything, just each invidiual page in the list that needs it, so we only want to do one at a time. I strongly suspect it's a single base page each time, but for belts + braces I'm doing compound_nr(). See below, this is wrong, it should just be '1'. So this is iterating through a list of pagerefs that can be in any random order. > > (something that's always troubled me about compound_nr() is that it > returns 1 for tail pages and the number you actually expect for head > pages) > OK I changed this from '1' to compound_nr() out of an (apparently) abundance of caution, but I was wrong: npagerefs = DIV_ROUND_UP(info->fix.smem_len, PAGE_SIZE); There are page refs for each PAGE_SIZE (i.e. base page size), so there is no way anything is compound. Will switch this to 1.
