On Wed, 2010-09-15 at 12:18 -0700, Hugh Dickins wrote: > On Wed, Sep 15, 2010 at 10:24 AM, Matt Mackall <mpm@xxxxxxxxxxx> wrote: > > > But that's my point: the consistency problem is NOT in smaps. The page > > is NOT marked dirty, ergo smaps doesn't report it as dirty. Whether or > > not there is MORE information smaps could be reporting is irrelevant, > > the information it IS reporting is consistent with the underlying VM > > data. If there's an inconsistency about what it means to be clean, it's > > either in the VM or in your head. > > > > And I frankly think it's in the VM. > > I don't believe there's any problem in the VM here, we'd be having > SIGSEGVs all over if there were. Of course it works. It's just not as orthogonal (aka consistent) as it could be in this case: it's not actually reflecting any of the usual meanings of dirtiness here. > The problem is that /proc/pid/smaps exports a simplified view of the > VM, and Richard and Nikanth were hoping that it gave them some info > which it has never pretended to give them, > > It happens to use a pte_dirty(ptent) test: you could argue that that > should be pte_dirty(ptent) || PageDirty(page) (which would then "fix > the issue" which Richard sees with swapoff/swapon), That might be interesting. Are there any other notable cases where pte_dirty() differs from PageDirty()? > or you could argue > that it should be pte_dirty(ptent) || PageDirty(page) || > PageSwapCache(page) (which would then note clean copies of swap cache > as dirty in the sense which Richard and Nikanth are interested in). > > But after these years, we should probably assume that most users of > /proc/pid/smaps are used to the existing pte_dirty(ptent) test, and > would be troubled by a departure from it. Dunno, my smem tool[1] is probably also expecting too much here and I should know better! > > In any case, I don't think Nikanth's fix is the right fix, as it > > basically says "you can't trust any of this". Either swap should return > > the pages to their pre-swap dirty state in the VM, or we should add > > another field here: > > > > Weird_Anon_Page_You_Should_Pretend_Is_Private_Dirty: 8 kB > > I think that the most widely useful but simple extension of > /proc/pid/smaps, that would give them the info they want, would indeed > be to counts ptes pointing to PageAnon pages and report that total on > an additional line (say, just before "Swap:"); but there's no need for > the derogatory name you propose there, "Anon:" would suit fine! Yes, that wasn't a serious suggestion. [1] http://www.selenic.com/smem/ -- Mathematics is the supreme nostalgia of our time. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>