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>> length = XFS_FSB_TO_B(mp, end_fsb) - offset; >> } >> >> - if (imap_needs_alloc(inode, flags, &imap, nimaps)) >> + needs_alloc = imap_needs_alloc(inode, flags, &imap, nimaps); >> + >> + if (flags & IOMAP_ATOMIC) { >> + error = -EAGAIN; >> + /* >> + * If we allocate less than what is required for the write >> + * then we may end up with multiple mappings, which means that >> + * REQ_ATOMIC-based cannot be used, so avoid this possibility. >> + */ >> + if (needs_alloc && orig_end_fsb - offset_fsb > 1) >> + goto out_unlock; > > I have a quick question here. Based on above check it looks like > allocation requests on a hole or the 1st time allocation (append writes) > for a given logical range will always be done using CoW fallback > mechanism, isn't it? Right, but... > So that means HW based multi-fsblock atomic write > request will only happen for over writes (non-discontigous extent), > correct? For an unwritten pre-allocated extent, we can use the REQ_ATOMIC method. fallocate (without ZERO RANGE) would give a pre-allocated unwritten extent, and a write there would not technically be an overwrite. > > Now, it's not always necessary that if we try to allocate an extent for > the given range, it results into discontiguous extents. e.g. say, if the > entire range being written to is a hole or append writes, then it might > just allocate a single unwritten extent which is valid for doing an > atomic write using HW/BIOs right? Right > And it is valid to write using unwritten extent as long as we don't have > mixed mappings i.e. the entire range should either be unwritten or > written for the atomic write to be untorned, correct? > We can't write to discontiguous extents, and a mixed mapping would mean discontiguous extents. And, as mentioned earlier, it is ok to use REQ_ATOMIC method on an unwritten extent. > I am guessing this is kept intentional? > Yes Thanks, John