On 23/01/31 06:48PM, Matthew Wilcox wrote: > On Wed, Feb 01, 2023 at 12:07:25AM +0530, Ritesh Harjani (IBM) wrote: > > On 23/01/30 09:00PM, Matthew Wilcox wrote: > > > On Tue, Jan 31, 2023 at 01:51:50AM +0530, Ritesh Harjani (IBM) wrote: > > > > > > Thus the iop structure will only gets allocated at the time of writeback > > > > > > in iomap_writepage_map(). This I think, was a not problem till now since > > > > > > we anyway only track uptodate status in iop (no support of tracking > > > > > > dirty bitmap status which later patches will add), and we also end up > > > > > > setting all the bits in iomap_page_create(), if the page is uptodate. > > > > > > > > > > delayed iop allocation is a feature and not a bug. We might have to > > > > > refine the criteria for sub-page dirty tracking, but in general having > > > > > the iop allocates is a memory and performance overhead and should be > > > > > avoided as much as possible. In fact I still have some unfinished > > > > > work to allocate it even more lazily. > > > > > > > > So, what I meant here was that the commit[1] chaged the behavior/functionality > > > > without indenting to. I agree it's not a bug. > > > > > > It didn't change the behaviour or functionality. It broke your patches, > > > but it certainly doesn't deserve its own commit reverting it -- because > > > it's not wrong. > > > > > > > But when I added dirty bitmap tracking support, I couldn't understand for > > > > sometime on why were we allocating iop only at the time of writeback. > > > > And it was due to a small line change which somehow slipped into this commit [1]. > > > > Hence I made this as a seperate patch so that it doesn't slip through again w/o > > > > getting noticed/review. > > > > > > It didn't "slip through". It was intended. > > > > > > > Thanks for the info on the lazy allocation work. Yes, though it is not a bug, but > > > > with subpage dirty tracking in iop->state[], if we end up allocating iop only > > > > at the time of writeback, than that might cause some performance degradation > > > > compared to, if we allocat iop at ->write_begin() and mark the required dirty > > > > bit ranges in ->write_end(). Like how we do in this patch series. > > > > (Ofcourse it is true only for bs < ps use case). > > > > > > > > [1]: https://lore.kernel.org/all/20220623175157.1715274-5-shr@xxxxxx/ > > > > > > You absolutely can allocate it in iomap_write_begin, but you can avoid > > > allocating it until writeback time if (pos, len) entirely overlap the > > > folio. ie: > > > > > > if (pos > folio_pos(folio) || > > > pos + len < folio_pos(folio) + folio_size(folio)) > > > iop = iomap_page_create(iter->inode, folio, iter->flags, false); > > > > Thanks for the suggestion. However do you think it will be better if this is > > introduced along with lazy allocation changes which Christoph was mentioning > > about? > > Why I am thinking that is because, with above approach we delay the allocation > > of iop until writeback, for entire folio overlap case. But then later > > in __iomap_write_begin(), we require iop if folio is not uptodate. > > Hence we again will have to do some checks to see if the iop is not allocated > > then allocate it (which is for entire folio overlap case). > > That somehow looked like an overkill for a very little gain in the context of > > this patch series. Kindly let me know your thoughts on this. > > Look at *why* __iomap_write_begin() allocates an iop. It's to read in the > blocks which are going to be partially-overwritten by the write. If the > write overlaps the entire folio, there are no parts which need to be read > in, and we can simply return. Yes that make sense. > Maybe we should make that more obvious: Yes, I think this maybe required. Because otherwise we might end up using uninitialized iop. generic/156 (funshare), can easily trigger that. Will spend sometime on the unshare path of iomap. > > if (folio_test_uptodate(folio)) > return 0; > if (pos <= folio_pos(folio) && > pos + len >= folio_pos(folio) + folio_size(folio)) > return 0; > folio_clear_error(folio); > > (I think pos must always be >= folio_pos(), so that <= could be ==, but > it doesn't hurt anything to use <=) Thanks for sharing this. -ritesh
