On Fri, Aug 12, 2011 at 12:15:21PM -0700, Hugh Dickins wrote: > On Fri, 12 Aug 2011, Minchan Kim wrote: > > On Fri, Aug 12, 2011 at 9:48 AM, Linus Torvalds > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Wed, Aug 10, 2011 at 11:40 PM, David Gibson > > > <david@xxxxxxxxxxxxxxxxxxxxx> wrote: > > >> > > >> This patch, therefore, stores a pointer to the inode instead of the > > >> address_space in the page private data for hugepages. More > > >> importantly it correctly adjusts the reference count on the inodes > > >> when they're added to the page private data. This ensures that the > > >> inode (and therefore the super block) will not be freed before we use > > >> it from free_huge_page. > > > > > > Looks sane, but I *really* want some acks from people who use/know > > > hugetlbfs. Who would that be? I'm adding random people who have > > > acked/signed-off patches to hugetlbfs recently.. > > > > At least, code itself looks good to me but your random choice was failed. > > Maybe people you want are as follows. > > http://marc.info/?t=126928975800003&r=1&w=2 > > > > Ccing right persons. > > I don't know much about hugetlbfs these days, but I think the patch > is very wrong. > > The real change is where alloc_huge_page() does igrab(inode) and > free_huge_pages() does iput(inode)? > > That makes me very nervous, partly because a final iput() is a complex > operation, which we wouldn't expect to be doing when "freeing" a page. > > My first worry was that free_huge_page() could actually get called at > interrupt time (when it's in a pagevec of pages to be freed as a batch, > then another put_page is done at interrupt time which frees that batch): > I worried that we use spin_lock not spin_lock_irqsave on inode->i_lock. > To be honest though, I've not followed up whether that's actually a > possibility, the compound page path is too twisty for a quick answer; > and even if it's a possibility, it's one that's already ignored in the > case of hugetlb_lock. > > Setting that aside, I think this thing of grabbing a reference to inode > for each page just does not work as you wish: when we unlink an inode, > all its pages should be freed; but because they are themselves holding > references to the inode, it and its pages stick around forever. Ugh, yes. You're absolutely right. That circular reference will mess everything up. Thinking it through and testing fail. > A quick experiment with your patch versus without confirmed that: > meminfo HugePages_Free stayed down with your patch, but went back to > HugePages_Total without it. Please check, perhaps I'm just mistaken. > > Sorry, I've not looked into what a constructive alternative might be; > and it's not the first time we've had this difficulty - it came up last > year when the ->freepage function was added, that the inode may be gone > by the time ->freepage(page) is called. Ok, so. In fact the quota functions we call at free time only need the super block, not the inode per se. If we put a superblock pointer instead of an inode pointer in page private, and refcounted that, I think that should remove the circular ref. The only reason I didn't do it before is that the superblock refcounting functions didn't seem to be globally visible in an obvious way. Does that sound like a reasonable approach? > On a side note, very good description - thank you, but I wish you'd > split the patch into two, the fix and then the inode-instead-of-mapping > cleanup. Though personally I'd prefer not to make that "cleanup": it's > normal for a struct address space * to be used in struct page (if I delved > I guess I'd find good reason why this one is in page->private instead of > page->mapping: perhaps because it's needed after page->mapping is reset > to NULL, perhaps because it's needed on COWed copies of hugetlbfs pages). That is an interesting question. But it doesn't address the basic point. mappings aren't refcounted themselves, and as far as I can tell their lifetime is bound to that of their inode. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
