On Wed, 2007-03-07 at 14:36 +0100, Nick Piggin wrote: > On Wed, Mar 07, 2007 at 02:19:22PM +0100, Peter Zijlstra wrote: > > On Wed, 2007-03-07 at 14:08 +0100, Nick Piggin wrote: > > > > > > > The thing is, I don't think anybody who uses these things cares > > > > > about any of the 'problems' you want to fix, do they? We are > > > > > interested in dirty pages only for the correctness issue, rather > > > > > than performance. Same as reclaim. > > > > > > > > If so, we can just stick to the dead slow but correct 'scan the full > > > > vma' page_mkclean() and nobody would ever trigger it. > > > > > > Not if we restricted it to root and mlocked tmpfs. But then why > > > wouldn't you just do it with the much more efficient msync walk, > > > so that if root does want to do writeout via these things, it does > > > not blow up? > > > > This is all used on ram based filesystems right, they all have > > BDI_CAP_NO_WRITEBACK afaik, so page_mkclean will never get called > > anyway. Mlock doesn't avoid getting page_mkclean called. > > > > Those who use this on a 'real' filesystem will get hit in the face by a > > linear scanning page_mkclean(), but AFAIK nobody does this anyway. > > But somebody might do it. I just don't know why you'd want to make > this _worse_ when the msync option would work? > > > Restricting it to root for such filesystems is unwanted, that'd severely > > handicap both UML and Oracle as I understand it (are there other users > > of this feature around?) > > Why? I think they all use tmpfs backings, don't they?
Ooh, you only want to restrict remap_file_pages on mappings from bdi's without BDI_CAP_NO_WRITEBACK. Sure, I can live with that, and I suspect others can as well. > > msync() might never get called and then we're back with the old > > behaviour where we can surprise the VM with a ton of dirty pages. > > But we're root. With your patch, root *can't* do nonlinear writeback > well. Ever. With msync, at least you give them enough rope. True. We could even guesstimate the nonlinear dirty pages by subtracting the result of page_mkclean() from page_mapcount() and force an msync(MS_ASYNC) on said mapping (or all (nonlinear) mappings of the related file) when some threshold gets exceeded. > > > > What is the DoS scenario wrt reclaim? We really ought to fix that if > > > > real, those UML farms run on nothing but nonlinear reclaim I'd think. > > > > > > I guess you can just increase the computational complexity of > > > reclaim quite easily. > > > > Right, on first glance it doesn't look to be too bad, but I should take > > a closer look. > > Well I don't think UML uses nonlinear yet anyway, does it? Can they > make do with restricting nonlinear to mlocked vmas, I wonder? Probably > not. I think it does, but lets ask, Jeff? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
