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List-Post: List-Help: List-Subscribe: , Cc: "fam@euphon.net" , "kwolf@redhat.com" , "qemu-devel@nongnu.org" , Denis Lunev Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 8/6/19 8:39 AM, Vladimir Sementsov-Ogievskiy wrote: > 03.08.2019 0:19, Max Reitz wrote: >> On 02.08.19 20:58, Vladimir Sementsov-Ogievskiy wrote: >>> hbitmap_reset is broken: it rounds up the requested region. It leads = to >>> the following bug, which is shown by fixed test: >>> >>> assume granularity =3D 2 >>> set(0, 3) # count becomes 4 >>> reset(0, 1) # count becomes 2 >>> >>> But user of the interface assume that virtual bit 1 should be still >>> dirty, so hbitmap should report count to be 4! >>> >>> In other words, because of granularity, when we set one "virtual" bit= , >>> yes, we make all "virtual" bits in same chunk to be dirty. But this >>> should not be so for reset. >>> >>> Fix this, aligning bound correctly. >>> >>> Signed-off-by: Vladimir Sementsov-Ogievskiy >>> --- >>> >>> Hi all! >>> >>> Hmm, is it a bug or feature? :) >>> I don't have a test for mirror yet, but I think that sync mirror may = be broken >>> because of this, as do_sync_target_write() seems to be using unaligne= d reset. >> >> Crap. >> >> >> Yes, you=E2=80=99re right. This would fix it, and it wouldn=E2=80=99t= fix it in the >> worst way. >> >> But I don=E2=80=99t know whether this patch is the best way forward st= ill. I >> think call hbitmap_reset() with unaligned boundaries generally calls f= or >> trouble, as John has laid out. If mirror=E2=80=99s do_sync_target_wri= te() is >> the only offender right now,=20 >=20 > Another thing is migration/block. Should we care of it, is it supported= at all? >=20 Downstream products always have time and room to get additional fixes; I think this is supported from an upstream POV so we should investigate thi= s. I assume migration/block has the same problem that it fully clears unaligned blocks? (I can look shortly but can't just yet.) --js