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 te/9ZSmd 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When the tree changes, the entry we've stored is wildly > wrong, but xas_reset() doesn't undo any of that. So when we actually > use the xas->xa_alloc nodes we've setup, they are pointing to the > wrong things. > > Which is probably why the commits in 6.10 added this: > > /* entry may have changed before we re-acquire the lock */ > if (alloced_order && (old != alloced_shadow || order != alloced_order)) { > xas_destroy(&xas); > alloced_order = 0; > } > > The only way to undo the work done by xas_split_alloc() is to call > xas_destroy(). > > To prove this theory, I tried making a minimal version that also > called destroy, but it all ended up less minimal than the code > that's actually in 6.10. I've got a long test going now with > an extra cond_resched() to make the race bigger, and a printk of victory. > > It hasn't fired yet, and I need to hop on an airplane, so I'll just leave > it running for now. But long story short, I think we should probably > just tag all of these for stable: > > https://lore.kernel.org/all/20240415171857.19244-2-ryncsn@gmail.com/T/#mdb85922624c39ea7efb775a044af4731890ff776 > > Also, Willy's proposed changes to xas_split_alloc() seem like a good > idea. A few days of load later and some extra printks, it turns out that taking the writer lock in __filemap_add_folio() makes us dramatically more likely to just return EEXIST than go into the xas_split_alloc() dance. With the changes in 6.10, we only get into that xas_destroy() case above when the conflicting entry is a shadow entry, so I changed my repro to use memory pressure instead of fadvise. I also added a schedule_timeout(1) after the split alloc, and with all of that I'm able to consistently make the xas_destroy() case trigger without causing any system instability. Kairui Song's patches do seem to have fixed things nicely. -chris