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.
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