On Mon, Jun 01, 2015 at 04:10:46PM -0700, John Hubbard wrote: > On Mon, 1 Jun 2015, Jerome Glisse wrote: > > On Fri, May 29, 2015 at 08:43:59PM -0700, John Hubbard wrote: > > > On Thu, 21 May 2015, j.glisse@xxxxxxxxx wrote: > > > > From: Jérôme Glisse <jglisse@xxxxxxxxxx> [...] > > > > + MMU_ISDIRTY, > > > > > > This MMU_ISDIRTY seems like a problem to me. First of all, it looks > > > backwards: the only place that invokes it is the clear_refs_write() > > > routine, for the soft-dirty tracking feature. And in that case, the pages > > > are *not* being made dirty! Rather, the kernel is actually making the > > > pages non-writable, in order to be able to trap the subsequent page fault > > > and figure out if the page is in active use. > > > > > > So, given that there is only one call site, and that call site should > > > actually be setting MMU_WRITE_PROTECT instead (I think), let's just delete > > > MMU_ISDIRTY. > > > > > > Come to think about it, there is no callback possible for "a page became > > > dirty", anyway. Because the dirty and accessed bits are actually set by > > > the hardware, and software is generally unable to know the current state. > > > So MMU_ISDIRTY just seems inappropriate to me, across the board. > > > > > > I'll take a look at the corresponding HMM_ISDIRTY, too. > > > > Ok i need to rename that one to CLEAR_SOFT_DIRTY, the idea is that > > for HMM i would rather not write protect the memory for the device > > and just rely on the regular and conservative dirtying of page. The > > soft dirty is really for migrating a process where you first clear > > the soft dirty bit, then copy memory while process still running, > > then freeze process an only copy memory that was dirtied since > > first copy. Point being that adding soft dirty to HMM is something > > that can be done down the road. We should have enough bit inside > > the device page table for that. > > > > Yes, I think renaming it to CLEAR_SOFT_DIRTY will definitely allow more > accurate behavior in response to these events. > > Looking ahead, a couple things: > > 1. This mechanism is also used for general memory utilization tracking (I > see that Vladimir DavyDov has an "idle memory tracking" proposal that > assumes this works, for example: https://lwn.net/Articles/642202/ and > https://lkml.org/lkml/2015/5/12/449). > > 2. It seems hard to avoid the need to eventually just write protect the > page, whether it is on the CPU or the remote device, if things like device > drivers or user space need to track write accesses to a virtual address. > Either you write protect the page, and trap the page faults, or you wait > until later and read the dirty bit (indirectly, via something like > unmap_mapping_range). Or did you have something else in mind? > > Anyway, none of that needs to hold up this part of the patchset, because > the renaming fixes things up for the future code to do the right thing. I will go over Vladimir patchset it was on my radar but haven't had yet a chance to go over it. We will likely need to do the write protect for device too. But as you said this is not an issue that this patch need a fix for, only HMM would need to change. I will do that. [...] > > > We may have to add MMU_READ_WRITE (and maybe another one, I haven't > > > bottomed out on that), if you agree with the above approach of > > > always sending a precise event, instead of "protection changed". > > > > I think Linus point made sense last time, but i would need to read > > again the thread. The idea of that patch is really to provide context > > information on what kind of CPU page table changes is happening and > > why. > > > > Shoot, I tried to find that conversation, but my search foo is too weak. > If you have a link to that thread, I'd appreciate it, so I can refresh my > memory. > > I was hoping to re-read it and see if anything has changed. It's not > really a huge problem to call find_vma() again, but I do want to be sure > that there's a good reason for doing so. > > Otherwise, I'll just rely on your memory that Linus preferred your current > approach, and call it good, then. http://lkml.iu.edu/hypermail/linux/kernel/1406.3/04880.html I am working on doing some of the changes discussed so far, i will push my tree to git://people.freedesktop.org/~glisse/linux hmm branch once i am done. Cheers, Jérôme -- 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>