Hi Darrick, On Wed, Jul 21, 2021 at 03:24:04PM -0700, Darrick J. Wong wrote: > On Wed, Jul 21, 2021 at 04:23:23PM +0800, Gao Xiang wrote: > > This tries to add tail packing inline read to iomap, which can support > > several inline tail blocks. Similar to the previous approach, it cleans > > post-EOF in one iteration. > > > > The write path remains untouched since EROFS cannot be used for testing. > > It'd be better to be implemented if upcoming real users care rather than > > leave untested dead code around. > > I had a conversation with Gao on IRC this morning, and I think I've > finally gotten up to speed on where he's trying to go with this > patchset. Maybe that will make review of this patch easier, or at least > not muddy the waters further. Many thanks for your time on this and the detailed long reply. > > Right now, inline data in iomap serves exactly two users -- gfs2 and > ext4. ext4 doesn't use iomap for buffered IO and doesn't support > directio for inline data files, so we can ignore them for now. gfs2 > uses iomap for buffered IO, and it stores the inline data after the > gfs2_dinode. > > iomap's inline data functions exist to serve the gfs2 use case, which is > why the code has baked-in assumptions that iomap->offset is always zero, > and the length is never more than a page. > Yeah, that is why I need to update the iomap inline code before convering all buffer I/O stuffs to iomap. > It used to be the case that we'd always attach an iomap_page to a page > for blocksize < pagesize files, but as of 5.14-rc2 we're starting to > move towards creating and dropping them on demand. IOWs, reads from > inline data files always read the entire file contents into the page so > we mark the whole page uptodate and do not attach an iomap_page (unlike > regular reads). Writes don't attach an iomap_page to inline data files. > Writeback attaches an iomap_page. Yeah. > > Did I get that much right? Onto the erofs part, now that I've also > taken the time to figure out what it's doing by reading the ondisk > format in Documentation/. (Thanks for that, erofs developers!) > > erofs can perform tail packing to reduce internal block fragmentation. > Tails of files are written immediately after the ondisk inode, which is > why Gao wants to use IOMAP_INLINE for this. Note that erofs tailpacking > is /not/ same as what reiserfs does, and the inlinedata model is /not/ > the same as what gfs2 does. > > A tail-packed erofs file mapping looks like this: > > x = round_down(i_size, blocksize); > [0..(x - 1)]: mapped to a range of external blocks > [x..(i_size - 1)]: inline data immediately after the inode > Correct. > The previous discussions have gone a bit afield -- there's only one > inline data region per file, it won't cross a page boundary because > erofs requires blocksize == pagesize, and it's always at the end of the > file. I don't know how we got onto the topic of multiple inline data > regions or encoded regions in the middle of a file, but that's not on > anybody's requirement list today, AFAICT. Sorry, I just tried to give an example. And I saw it misled the topic, very sorry about that. > > I suspect that adapting the inlinedata code to support regions that > don't start at offset zero but are otherwise page-aligned can be done > with fairly minor changes to the accounting, since I think that largely > can be done by removing the asserts about offset==0. I think I could try to figure out a page-size aligned only patch like this, as long as each one is happy about that. > > Did I get that right? > > The next thing the erofs developers want to do is add support for > blocksize < pagesize, presumably so that they can mount a 4k blocksize > erofs volume on a machine with 64k pages. For that, I think erofs needs > to be able to read the tail bytes into the middle of an existing page. > Hence the need to update the per-block uptodate bits in the iomap_page > from the read function, and all the math changes where we increase the > starting address of a copy by (iomap->offset - pos). The end result > should be that we can handle inline data regions anywhere, though we > won't really have a way to test that until erofs starts supporting > blocksize < pagesize. Correct. > > Assuming that my assumptions are correct, I think this patch decomposes > into three more targeted changes, one of which applies now, and the rest > which will go with the later effort. > > 1) Update the code to handle inline data mappings where iomap->offset is > not zero but the start of the mapping is always page-aligned. I could write a patch just for page-aligned cases for now. > > 2) Adapt the inline data code to create and update the iop as > appropriate. This could be a little tricky since I've seen elsewhere in > the v4 discussion thread that people like the idea of not paying the iop > overhead for pages that are backed by a single extent even when bs < ps. > I suspect we have enough to decide this from the *iomap/*srcmap length > in iomap_readpage_actor or iomap_write_begin, though I've not written > any code that tries that. > > 3) Update the code to handle inline data mappings where iomap->offset > can point to the middle of a page. Ok, so I think I will unprioritize 2) and 3) for now. and just address the page-aligned approach since it's currently what EROFS needs. > > My apologies if everyone else already figured all of this out; for all I > know I'm merely scrawling this here as notes to refer back to for future > discussions. > > > Cc: Christoph Hellwig <hch@xxxxxx> > > Cc: Darrick J. Wong <djwong@xxxxxxxxxx> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > > Cc: Andreas Gruenbacher <andreas.gruenbacher@xxxxxxxxx> > > Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> > > --- > > v4: https://lore.kernel.org/r/20210720133554.44058-1-hsiangkao@xxxxxxxxxxxxxxxxx > > changes since v4: > > - turn to WARN_ON_ONCE() suggested by Darrick; > > - fix size to "min(iomap->length + iomap->offset - pos, > > PAGE_SIZE - poff)" > > > > fs/iomap/buffered-io.c | 58 +++++++++++++++++++++++++++--------------- > > fs/iomap/direct-io.c | 13 +++++++--- > > 2 files changed, 47 insertions(+), 24 deletions(-) > > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > > index 87ccb3438bec..d8436d34a159 100644 > > --- a/fs/iomap/buffered-io.c > > +++ b/fs/iomap/buffered-io.c > > @@ -205,25 +205,27 @@ struct iomap_readpage_ctx { > > struct readahead_control *rac; > > }; > > > > -static void > > +static int > > iomap_read_inline_data(struct inode *inode, struct page *page, > > - struct iomap *iomap) > > + struct iomap *iomap, loff_t pos) > > { > > - size_t size = i_size_read(inode); > > + unsigned int size, poff = offset_in_page(pos); > > void *addr; > > > > - if (PageUptodate(page)) > > - return; > > - > > - BUG_ON(page_has_private(page)); > > - BUG_ON(page->index); > > - BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data)); > > + /* inline source data must be inside a single page */ > > + if (WARN_ON_ONCE(iomap->length > PAGE_SIZE - > > + offset_in_page(iomap->inline_data))) > > + return -EIO; > > + /* handle tail-packing blocks cross the current page into the next */ > > + size = min_t(unsigned int, iomap->length + iomap->offset - pos, > > + PAGE_SIZE - poff); > > Part of my confusion has resulted from this comment -- now that I think > I understand the problem domain better, I realize that the clamping code > here is not because erofs will hand us a tail-packing iomap that crosses > page boundaries; this clamp simply protects us from memory corruption. > > /* > * iomap->inline_data is a kernel-mapped memory page, so we must > * terminate the read at the end of that page. > */ > if (WARN_ON_ONCE(...)) > return -EIO; > size = min_t(...); That sounds much better. I will update like this. > > TBH I wonder if we merely need a rule that ->iomap_begin must not hand > back an inline data mapping that crosses a page, since I think the > check in the previous line is sufficient. > > > addr = kmap_atomic(page); > > - memcpy(addr, iomap->inline_data, size); > > - memset(addr + size, 0, PAGE_SIZE - size); > > + memcpy(addr + poff, iomap->inline_data - iomap->offset + pos, size); > > I keep seeing this (iomap->inline_data + pos - iomap->offset) > construction in this patch, maybe it should be a helper? I'm fine with this, (but I'm not good at naming), may I ask for some suggested naming? e.g. static inline void *iomap_adjusted_inline_data(iomap, pos) does that look good? > > > + memset(addr + poff + size, 0, PAGE_SIZE - poff - size); > > kunmap_atomic(addr); > > - SetPageUptodate(page); > > + iomap_set_range_uptodate(page, poff, PAGE_SIZE - poff); > > + return PAGE_SIZE - poff; > > } > > > > static inline bool iomap_block_needs_zeroing(struct inode *inode, > > @@ -245,19 +247,23 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data, > > loff_t orig_pos = pos; > > unsigned poff, plen; > > sector_t sector; > > + int ret; > > > > - if (iomap->type == IOMAP_INLINE) { > > - WARN_ON_ONCE(pos); > > - iomap_read_inline_data(inode, page, iomap); > > - return PAGE_SIZE; > > - } > > - > > - /* zero post-eof blocks as the page may be mapped */ > > iop = iomap_page_create(inode, page); > > + /* needs to skip some leading uptodate blocks */ > > iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen); > > if (plen == 0) > > goto done; > > > > + if (iomap->type == IOMAP_INLINE) { > > + ret = iomap_read_inline_data(inode, page, iomap, pos); > > + if (ret < 0) > > + return ret; > > + plen = ret; > > + goto done; > > + } > > + > > + /* zero post-eof blocks as the page may be mapped */ > > if (iomap_block_needs_zeroing(inode, iomap, pos)) { > > zero_user(page, poff, plen); > > iomap_set_range_uptodate(page, poff, plen); > > @@ -589,6 +595,18 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags, > > return 0; > > } > > > > +static int iomap_write_begin_inline(struct inode *inode, loff_t pos, > > + struct page *page, struct iomap *srcmap) > > +{ > > + /* needs more work for the tailpacking case, disable for now */ > > + if (WARN_ON_ONCE(srcmap->offset != 0)) > > + return -EIO; > > + if (PageUptodate(page)) > > + return 0; > > + iomap_read_inline_data(inode, page, srcmap, 0); > > + return 0; > > +} > > + > > static int > > iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags, > > struct page **pagep, struct iomap *iomap, struct iomap *srcmap) > > @@ -618,7 +636,7 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags, > > } > > > > if (srcmap->type == IOMAP_INLINE) > > - iomap_read_inline_data(inode, page, srcmap); > > + status = iomap_write_begin_inline(inode, pos, page, srcmap); > > else if (iomap->flags & IOMAP_F_BUFFER_HEAD) > > status = __block_write_begin_int(page, pos, len, NULL, srcmap); > > else > > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c > > index 9398b8c31323..cbadb99fb88c 100644 > > --- a/fs/iomap/direct-io.c > > +++ b/fs/iomap/direct-io.c > > @@ -379,22 +379,27 @@ iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length, > > { > > struct iov_iter *iter = dio->submit.iter; > > size_t copied; > > + void *dst = iomap->inline_data + pos - iomap->offset; > > > > - BUG_ON(pos + length > PAGE_SIZE - offset_in_page(iomap->inline_data)); > > + /* inline data must be inside a single page */ > > + if (WARN_ON_ONCE(length > PAGE_SIZE - > > + offset_in_page(iomap->inline_data))) > > + return -EIO; > > /* > * iomap->inline_data is a kernel-mapped memory page, so we must > * terminate the write at the end of that page. > */ > if (WARN_ON_ONCE(...)) > return -EIO; Ok. > > > if (dio->flags & IOMAP_DIO_WRITE) { > > I thought we weren't allowing writes to an inline mapping unless > iomap->offset == 0? Why is it necessary to change the directio write > path? Shouldn't this be: > > /* needs more work for the tailpacking case, disable for now */ > if (WARN_ON_ONCE(pos > 0)) > return -EIO; That is because Andreas once pointed out a case in: https://lore.kernel.org/r/CAHpGcMJ4T6byxqmO6zZF78wuw01twaEvSW5N6s90qWm0q_jCXQ@xxxxxxxxxxxxxx/ "This should be a WARN_ON_ONCE(srcmap->offset != 0). Otherwise, something like: xfs_io -ft -c 'pwrite 1 2' will fail because pos will be 1." I think that is reasonable to gfs2. So I changed like this. Thanks, Gao Xiang > > --D > > > loff_t size = inode->i_size; > > > > if (pos > size) > > - memset(iomap->inline_data + size, 0, pos - size); > > - copied = copy_from_iter(iomap->inline_data + pos, length, iter); > > + memset(iomap->inline_data + size - iomap->offset, > > + 0, pos - size); > > + copied = copy_from_iter(dst, length, iter); > > if (copied) { > > if (pos + copied > size) > > i_size_write(inode, pos + copied); > > mark_inode_dirty(inode); > > } > > } else { > > - copied = copy_to_iter(iomap->inline_data + pos, length, iter); > > + copied = copy_to_iter(dst, length, iter); > > } > > dio->size += copied; > > return copied; > > -- > > 2.24.4 > >
