On Sun, Mar 22, 2015 at 05:02:58PM -0700, Hugh Dickins wrote: > On Thu, 19 Mar 2015, Kirill A. Shutemov wrote: > > On Thu, Mar 19, 2015 at 11:29:52AM -0700, Dave Hansen wrote: > > > On 03/19/2015 10:08 AM, Kirill A. Shutemov wrote: > > > > The odd exception is PG_dirty: sound uses compound pages and maps them > > > > with PTEs. NO_COMPOUND triggers VM_BUG_ON() in set_page_dirty() on > > > > handling shared fault. Let's use HEAD for PG_dirty. > > It really depends on what you do with PageDirty of the head, when you > get to support 4k pagecache with subpages of a huge compound page. > > HEAD will be fine, so long as PageDirty on the head means the whole > huge page must be written back. I expect that's what you will choose; > but one could consider that if a huge page is only mapped read-only, > but a few subpages of it writable, then only the few need be written > back, in which case ANY would be more appropriate. NO_COMPOUND is > certainly wrong. > > But that does illustrate that I consider this patch series premature: > it belongs with your huge pagecache implementation. You seem to be > "tidying up" and adding overhead to things that are fine as they are. I agree, it can be ANY too, since we don't use PG_dirty anywhere at the moment. My first thought was that it's better to match PG_dirty behaviour with LRU-related, but it's not necessary should be the case. BTW, do we make any use of PG_dirty on pages with ->mapping == NULL? Should we avoid dirtying them in the first place? > > > Can we get the sound guys to look at this, btw? It seems like an odd > > > thing that we probably don't want to keep around, right? > > > > CC: +sound guys > > I don't think this is peculiar to sound at all: there are other users > of __GFP_COMP in the tree, aren't there? And although some of them > might turn out not to need it any more, I expect most of them still > need it for the same reason they did originally. I haven't seen any other __GFP_COMP user which get it mapped to user-space with PTEs. Do you? Probably I haven't just stepped on it. ... looking into code a bit more: at least one fb-drivers has compound pages mapped with PTEs.. > > I'm not sure what is right fix here. At the time adding __GFP_COMP was a > > fix: see f3d48f0373c1. > > The only thing special about this one, was that I failed to add > __GFP_COMP at first. > > The purpose of __GFP_COMP is to allow a >0-order page (originally, just > a hugetlb page: see 2.5.60) to be mapped into userspace, and parts of it > then subjected to get_user_pages (ptrace, futex, direct I/O, infiniband > etc), and now even munmap, without destroying the integrity of the > underlying >0-order page. > > We don't bother with __GFP_COMP when a >0-order page cannot be mapped > into userspace (except through /dev/mem or suchlike); we add __GFP_COMP > when it might be, to get the right reference counting. Wouldn't non-compound >0-order page allocation + split_page() work too? > It's normal for set_page_dirty() to be called in the course of > get_user_pages(), and it's normal for set_page_dirty() to be called > when releasing the get_user_pages() references, and it's normal for > set_page_dirty() to be called when munmap'ing a pte_dirty(). > > > > > Other odd part about __GFP_COMP here is that we have ->_mapcount in tail > > pages to be used for both: mapcount of the individual page and for gup > > pins. __compound_tail_refcounted() doesn't recognize that we don't need > > tail page accounting for these pages. > > So page->_mapcount of the tails is being used for both their mapcount > and their reference count: that's certainly funny, and further reason > to pursue your aim of simplifying the way THPs are refcounted. But > not responsible for any actual bug, I think? GUP pin would screw up page_mapcount() on these pages. It would affect memory stats for the process and probably something else. I think we can get __compound_tail_refcounted() ignore these pages by checking if page->mapping is NULL. > > Hugh, I tried to ask you about the situation several times (last time on > > the summit). Any comments? > > I do remember we began a curtailed conversation about this at LSF/MM. > I do not remember you asking about it earlier: when was that? http://lkml.kernel.org/g/20141217004734.GA23150@xxxxxxxxxxxxxxxxx -- Kirill A. Shutemov -- 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>