On Tue, Nov 17, 2020 at 08:26:03AM -0800, Hugh Dickins wrote: > On Tue, 17 Nov 2020, Matthew Wilcox wrote: > > On Mon, Nov 16, 2020 at 02:34:34AM -0800, Hugh Dickins wrote: > > > Fix to [PATCH v4 15/16] mm/truncate,shmem: Handle truncates that split > > > THPs. > > > One machine ran fine, swapping and building in ext4 on loop0 on huge > > > tmpfs; > > > one machine got occasional pages of zeros in its .os; one machine couldn't > > > get started because of ext4_find_dest_de errors on the newly mkfs'ed fs. > > > The partial_end case was decided by PAGE_SIZE, when there might be a THP > > > there. The below patch has run well (for not very long), but I could > > > easily have got it slightly wrong, off-by-one or whatever; and I have > > > not looked into the similar code in mm/truncate.c, maybe that will need > > > a similar fix or maybe not. > > > > Thank you for the explanation in your later email! There is indeed an > > off-by-one, although in the safe direction. > > > > > --- 5103w/mm/shmem.c 2020-11-12 15:46:21.075254036 -0800 > > > +++ 5103wh/mm/shmem.c 2020-11-16 01:09:35.431677308 -0800 > > > @@ -874,7 +874,7 @@ static void shmem_undo_range(struct inod > > > long nr_swaps_freed = 0; > > > pgoff_t index; > > > int i; > > > - bool partial_end; > > > + bool same_page; > > > > > > if (lend == -1) > > > end = -1; /* unsigned, so actually very big */ > > > @@ -907,16 +907,12 @@ static void shmem_undo_range(struct inod > > > index++; > > > } > > > > > > - partial_end = ((lend + 1) % PAGE_SIZE) > 0; > > > + same_page = (lstart >> PAGE_SHIFT) == end; > > > > 'end' is exclusive, so this is always false. Maybe something "obvious": > > > > same_page = (lstart >> PAGE_SHIFT) == (lend >> PAGE_SHIFT); > > > > (lend is inclusive, so lend in 0-4095 are all on the same page) > > My brain is not yet in gear this morning, so I haven't given this the > necessary thought: but I do have to question what you say there, and > throw it back to you for the further thought - > > the first shmem_getpage(inode, lstart >> PAGE_SHIFT, &page, SGP_READ); > the second shmem_getpage(inode, end, &page, SGP_READ). > So same_page = (lstart >> PAGE_SHIFT) == end > had seemed right to me.
I find both of these functions exceptionally confusing. Does this make it easier to understand? @@ -859,22 +859,47 @@ static void shmem_undo_range(struct inode *inode, loff_t l start, loff_t lend, { struct address_space *mapping = inode->i_mapping; struct shmem_inode_info *info = SHMEM_I(inode); - pgoff_t start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT; - pgoff_t end = (lend + 1) >> PAGE_SHIFT; + pgoff_t start = lstart >> PAGE_SHIFT; + pgoff_t end = lend >> PAGE_SHIFT; struct pagevec pvec; pgoff_t indices[PAGEVEC_SIZE]; struct page *page; long nr_swaps_freed = 0; pgoff_t index; int i; - bool same_page; + bool same_page = (start == end); - if (lend == -1) - end = -1; /* unsigned, so actually very big */ + page = NULL; + shmem_getpage(inode, start, &page, SGP_READ); + if (page) { + page = thp_head(page); + same_page = lend < page_offset(page) + thp_size(page); + set_page_dirty(page); + if (truncate_inode_partial_page(page, lstart, lend)) + start++; + else + start = page->index + thp_nr_pages(page); + unlock_page(page); + put_page(page); + page = NULL; + } + + if (!same_page) + shmem_getpage(inode, end, &page, SGP_READ); + if (page) { + page = thp_head(page); + set_page_dirty(page); + if (truncate_inode_partial_page(page, lstart, lend)) + end--; + else + end = page->index - 1; + unlock_page(page); + put_page(page); + } pagevec_init(&pvec); index = start; - while (index < end && find_lock_entries(mapping, index, end - 1, + while (index <= end && find_lock_entries(mapping, index, end, &pvec, indices)) { for (i = 0; i < pagevec_count(&pvec); i++) { page = pvec.pages[i]; @@ -900,40 +925,11 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend, index++; } - same_page = (lend >> PAGE_SHIFT) == (lstart >> PAGE_SHIFT); - page = NULL; - shmem_getpage(inode, lstart >> PAGE_SHIFT, &page, SGP_READ); - if (page) { - page = thp_head(page); - same_page = lend < page_offset(page) + thp_size(page); - set_page_dirty(page); - if (!truncate_inode_partial_page(page, lstart, lend)) { - start = page->index + thp_nr_pages(page); - if (same_page) - end = page->index; - } - unlock_page(page); - put_page(page); - page = NULL; - } - - if (!same_page) - shmem_getpage(inode, end, &page, SGP_READ); - if (page) { - page = thp_head(page); - set_page_dirty(page); - if (!truncate_inode_partial_page(page, lstart, lend)) - end = page->index; - unlock_page(page); - put_page(page); - } - index = start; - while (index < end) { + while (index <= end) { cond_resched(); - if (!find_get_entries(mapping, index, end - 1, &pvec, - indices)) { + if (!find_get_entries(mapping, index, end, &pvec, indices)) { /* If all gone or hole-punch or unfalloc, we're done */ if (index == start || end != -1) break; That is, we change the definitions of start and end to be the more natural "index of page which contains the first/last byte". Then we deal with the start and end of the range, and adjust the start & end appropriately. I almost managed to get rid of 'same_page' until I thought about the case where start was a compound page, and split succeeded. In this case, we already dealt with the tail and don't want to deal with it again.