On Wed, Jan 08, 2025 at 07:41:31PM +0000, Lorenzo Stoakes wrote: > 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. Actually, reading through the code, I think the comment is with regards to page_mkwrite(), so we should be ok, in fb_deferred_io_track_page(): /* * We want the page to remain locked from ->page_mkwrite until * the PTE is marked dirty to avoid mapping_wrprotect_page() * being called before the PTE is updated, which would leave * the page ignored by defio. * Do this by locking the page here and informing the caller * about it with VM_FAULT_LOCKED. */ lock_page(pageref->page); I don't think we need to lock the page (which is managed as kernel memory so doesn't require it). So will remove. > > 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.
