On Tue, 24 Apr 2007 14:09:33 +0100 (BST) Hugh Dickins <[EMAIL PROTECTED]> wrote:
> On Mon, 23 Apr 2007, Andrew Morton wrote: > > > > OK. I hope. the mapping_gfp_mask() here will have come from bdget()'s > > mapping_set_gfp_mask(&inode->i_data, GFP_USER); If anyone is accidentally > > setting __GFP_HIGHMEM on a blockdev address_space we'll cause ghastly > > explosions. Albeit ones which were well-deserved. > > I've not yet looked at the patch under discussion, but this remark > prompts me... a couple of days ago I got very worried by the various > hard-wired GFP_HIGHUSER allocations in mm/migrate.c and mm/mempolicy.c, > and wondered how those would work out if someone has a blockdev mmap'ed. > > I tried to test it out before sending a patch, but found no problem at > all: maybe I was too timid (fearing to corrupt my whole system), maybe > I've forgotten how that stuff works and wasn't doing the right thing > to reproduce it (I was mmap'ing /dev/sdb1 readonly, at the same time > as having it mounted as ext2 - when I forced migration to random pages, > then cp'ed /dev/zero to reuse the old pages, I was expecting ext2 to > get very upset with its metadata; mmap'ing while mounted isn't very > realistic, but my earlier sequence hadn't shown any problem either, > so I thought the cache got invalidated in between). Yipes. > Here's the patch I'd suggest adding if you believe there really is > a problem there: it's far from ideal (I can imagine mapping_gfp_mask > being used to enforce other restrictions, but the __GFP_HIGHMEM issue > seems to be the only one in practice; and it would be a shame to > restrict all the architectures which have no concept of HIGHMEM). > If there's no such problem, sorry for wasting your time. Yes, I believe there is such a problem. We ignore the fact that the blockdev address_space doesn't implement ->migratepage and we cheerily call fallback_migrate_page(), which does the wrong thing. > (If vma->vm_file is non-NULL, we can be sure vma->vm_file->f_mapping > is non-NULL, can't we? Some common code assumes that, some does not: > I've avoided cargo-cult safety below, but don't let me make it unsafe.) > > > Is there a problem with page migration to HIGHMEM, if pages were > mapped from a GFP_USER block device? I failed to demonstrate any > problem, but here's a quick fix if needed. > > Signed-off-by: Hugh Dickins <[EMAIL PROTECTED]> > > --- 2.6.21-rc7/include/linux/migrate.h 2007-03-07 13:08:59.000000000 > +0000 > +++ linux/include/linux/migrate.h 2007-04-24 13:18:31.000000000 +0100 > @@ -2,6 +2,7 @@ > #define _LINUX_MIGRATE_H > > #include <linux/mm.h> > +#include <linux/pagemap.h> > > typedef struct page *new_page_t(struct page *, unsigned long private, int > **); > > @@ -10,6 +11,13 @@ static inline int vma_migratable(struct > { > if (vma->vm_flags & (VM_IO|VM_HUGETLB|VM_PFNMAP|VM_RESERVED)) > return 0; > +#ifdef CONFIG_HIGHMEM > + if (vma->vm_file) { > + struct address_space *mapping = vma->vm_file->f_mapping; > + if (!(mapping_gfp_mask(mapping) & __GFP_HIGHMEM)) > + return 0; > + } > +#endif > return 1; > } >From my reading it would be pretty simple to teach unmap_and_move() to pass mapping_gfp_mask(page_mapping(page)) down into (*get_new_page)() to get the correct type of page. - 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/