On Thu, Sep 18, 2014 at 03:58:26PM +0800, Qu Wenruo wrote:
>
[...]
> >
> >original: (start >= existing->start && start < extent_map_end(existing))
> >this patch: (start < extent_map_end(existing) && start + len > existing->start)
> >
> >(start + len > existing->start) doesn't equal to start >= existing->start,
> >here is a case of (start+len > existing->start) but (start <= existing->start).
> >
> > |--------| -->(existing)
> > |--------| -->[start, start+len)
> >
> >And calling search_extent_mapping() doesn't make sure that
> >(start >= existing->start) is true, either.
> All right, that case is right and I'm wrong.
> I'll change the if() to use start >= existing->start.
>
> BTW, Any other problem?
Others look good.
thanks,
-liubo
>
> Thanks,
> Qu
> >
> >>>And one of overlapping cases is (existing->start > start), ie. em->start > start, this is
> >>>against our rule of btrfs_get_extent,
> >>Nope again, this overlapping in fact is quite normal in multithread
> >>random read/write.
> >>The files's [0~16) is a preallocated one,
> >>Thread A:
> >> write [4K, 8K) into the file, but not committed yet.
> >> extent map tree contains [0,16K) only
> >>Thread B:
> >>btrfs_get_extent()
> >> the map_start is 8K, len is 4K as an example
> >> grab a large em, take [0,16K), since [4K,8K) write is not committed.
> >> comes to insert: btrfs_release_path(path);
> >>
> >>Thread A:
> >> [4K, 8K) is not committed
> >> the extent map is now [0, 4K) [4K, 8K) [8K, 16K).
> >>
> >>Thread B:
> >> goes to insert: add_extent_mapping()
> >> the [0,16K) is overlapping, and the returned existing one is [8K, 16K).
> >> which contains the [map_start, map_start + len).
> >So this's an example of existing->start == start (both are 8K), not
> >existing->start > start.
> >
> >See __extent_writepage_io(),
> >
> >{
> > ...
> > em = epd->get_extent(inode, page, pg_offset, cur,
> > end - cur + 1, 1);
> > if (IS_ERR_OR_NULL(em)) {
> > SetPageError(page);
> > ret = PTR_ERR_OR_ZERO(em);
> > break;
> > }
> > extent_offset = cur - em->start;
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^ it needs to be (em->start <= cur)
> >
> > ...
> >}
> >
> >thanks,
> >-liubo
> >
> >>>struct extent_map *btrfs_get_extent(...)
> >>>{
> >>> [...]
> >>> insert:
> >>> btrfs_release_path(path);
> >>> if (em->start > start || extent_map_end(em) <= start) {
> >>> btrfs_err(root->fs_info, "bad extent! em: [%llu %llu] passed
> >>> [%llu %llu]",
> >>> em->start, em->len, start, len);
> >>> err = -EIO;
> >>> goto out;
> >>> }
> >>> [...]
> >>>}
> >>>
> >>>thanks,
> >>>-liubo
> >>>
> >>>>+ /*
> >>>>+ * The existing extent map is the one nearest to
> >>>>+ * the [start, start + len) range which overlaps
> >>>>+ */
> >>>>+ err = merge_extent_mapping(em_tree, existing,
> >>>>+ em, start);
> >>>> free_extent_map(existing);
> >>>>- existing = NULL;
> >>>>- }
> >>>>- if (!existing) {
> >>>>- existing = lookup_extent_mapping(em_tree, em->start,
> >>>>- em->len);
> >>>>- if (existing) {
> >>>>- err = merge_extent_mapping(em_tree, existing,
> >>>>- em, start);
> >>>>- free_extent_map(existing);
> >>>>- if (err) {
> >>>>- free_extent_map(em);
> >>>>- em = NULL;
> >>>>- }
> >>>>- } else {
> >>>>- err = -EIO;
> >>>>+ if (err) {
> >>>> free_extent_map(em);
> >>>> em = NULL;
> >>>> }
> >>>>--
> >>>>2.1.0
> >>>>
> >>>>--
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>
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