On Wed, Jul 24, 2013 at 10:36 AM, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 2013-07-24 at 21:17 +0400, Cyrill Gorcunov wrote: >> On Wed, Jul 24, 2013 at 10:06:53AM -0700, Andy Lutomirski wrote: >> > > Hi Andy, if I understand you correctly "file-backed pages" are carried >> > > in pte with _PAGE_FILE bit set and the swap soft-dirty bit won't be >> > > used on them but _PAGE_SOFT_DIRTY will be set on write if only I've >> > > not missed something obvious (Pavel?). >> > >> > If I understand this stuff correctly, the vmscan code calls >> > try_to_unmap when it reclaims memory, which makes its way into >> > try_to_unmap_one, which clears the pte (and loses the soft-dirty bit). >> >> Indeed, I was so stareing into swap that forgot about files. I'll do >> a separate patch for that, thanks! > > Lets just be clear about the problem first: the vmscan pass referred to > above happens only on clean pages, so the soft dirty bit could only be > set if the page was previously dirty and got written back. Now it's an > exercise for the reader whether we want to reinstantiate a cleaned > evicted page for the purpose of doing an iterative migration or whether > we want to flip the page in the migrated entity to be evicted (so if it > gets referred to, it pulls in an up to date copy) ... assuming the > backing file also gets transferred, of course. I think I understand your distinction. Nonetheless, given the loss of the soft-dirty bit, the migration tool could fail to notice that the pages was dirtied and subsequently cleaned and evicted. I'm unconvinced that doing this on a per-PTE basis is the right way, though. I've long wanted a feature to efficiently see what changed on a filesystem by comparing, say, a hash tree. NTFS can do this (sort of), but I don't think that anything else can. I think that btrfs should be able to, but there's no API that I've ever seen. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>