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SubjectRE: Bad use of highmem with buffer_migrate_page?
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Hello,

On Thursday, July 05, 2012 11:28 AM Rabin Vincent wrote:

> On Sat, May 12, 2012 at 3:21 AM, Laura Abbott <lauraa@codeaurora.org> wrote:
> > On 5/11/2012 1:30 AM, Marek Szyprowski wrote:
> >> On Thursday, May 10, 2012 10:08 PM Laura Abbott wrote:
> >>> I did a backport of the Contiguous Memory Allocator to a 3.0.8 tree. I
> >>> wrote fairly simple test case that, in 1MB chunks, allocs up to 40MB
> >>> from a reserved area, maps, writes, unmaps and then frees in an infinite
> >>> loop. When running this with another program in parallel to put some
> >>> stress on the filesystem, I hit data aborts in the filesystem/journal
> >>> layer, although not always the same backtrace. As an example:
> >>>
> >>> [<c02907a4>] (__ext4_check_dir_entry+0x20/0x184) from [<c029e1a8>]
> >>> (add_dirent_to_buf+0x70/0x2ac)
> >>> [<c029e1a8>] (add_dirent_to_buf+0x70/0x2ac) from [<c029f3f0>]
> >>> (ext4_add_entry+0xd8/0x4bc)
> >>> [<c029f3f0>] (ext4_add_entry+0xd8/0x4bc) from [<c029fe90>]
> >>> (ext4_add_nondir+0x14/0x64)
> >>> [<c029fe90>] (ext4_add_nondir+0x14/0x64) from [<c02a04c4>]
> >>> (ext4_create+0xd8/0x120)
> >>> [<c02a04c4>] (ext4_create+0xd8/0x120) from [<c022e134>]
> >>> (vfs_create+0x74/0xa4)
> >>> [<c022e134>] (vfs_create+0x74/0xa4) from [<c022ed3c>]
> >>> (do_last+0x588/0x8d4)
> >>> [<c022ed3c>] (do_last+0x588/0x8d4) from [<c022fe64>]
> >>> (path_openat+0xc4/0x394)
> >>> [<c022fe64>] (path_openat+0xc4/0x394) from [<c0230214>]
> >>> (do_filp_open+0x30/0x7c)
> >>> [<c0230214>] (do_filp_open+0x30/0x7c) from [<c0220cb4>]
> >>> (do_sys_open+0xd8/0x174)
> >>> [<c0220cb4>] (do_sys_open+0xd8/0x174) from [<c0105ea0>]
> >>> (ret_fast_syscall+0x0/0x30)
> >>>
> >>> Every panic had the same issue where a struct buffer_head [1] had a
> >>> b_data that was unexpectedly NULL.
> >>>
> >>> During the course of CMA, buffer_migrate_page could be called to migrate
> >>> from a CMA page to a new page. buffer_migrate_page calls set_bh_page[2]
> >>> to set the new page for the buffer_head. If the new page is a highmem
> >>> page though, the bh->b_data ends up as NULL, which could produce the
> >>> panics seen above.
> >>>
> >>> This seems to indicate that highmem pages are not not appropriate for
> >>> use as pages to migrate to. The following made the problem go away for
> >>> me:
> >>>
> >>> --- a/mm/page_alloc.c
> >>> +++ b/mm/page_alloc.c
> >>> @@ -5753,7 +5753,7 @@ static struct page *
> >>> __alloc_contig_migrate_alloc(struct page *page, unsigned long private,
> >>> int **resultp)
> >>> {
> >>> - return alloc_page(GFP_HIGHUSER_MOVABLE);
> >>> + return alloc_page(GFP_USER | __GFP_MOVABLE);
> >>> }
> >>>
> >>>
> >>> Does this seem like an actual issue or is this an artifact of my
> >>> backport to 3.0? I'm not familiar enough with the filesystem layer to be
> >>> able to tell where highmem can actually be used.
> >>
> >>
> >> I will need to investigate this further as this issue doesn't appear on
> >> v3.3+ kernels, but I remember I saw something similar when I tried CMA
> >> backported to v3.0.
>
> The problem is still present on latest mainline. The filesystem layer
> expects that the pages in the block device's mapping are not in highmem
> (the mapping's gfp mask is set in bdget()), but CMA replaces lowmem
> pages with highmem pages leading to the crashes.
>
> The above fix should work, but perhaps the following is preferable since
> it should allow moving highmem pages to other highmem pages?

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4403009..4a4f921 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5635,7 +5635,12 @@ static struct page *
> __alloc_contig_migrate_alloc(struct page *page, unsigned long private,
> int **resultp)
> {
> - return alloc_page(GFP_HIGHUSER_MOVABLE);
> + gfp_t gfp_mask = GFP_USER | __GFP_MOVABLE;
> +
> + if (PageHighMem(page))
> + gfp_mask |= __GFP_HIGHMEM;
> +
> + return alloc_page(gfp_mask);
> }
>
> /* [start, end) must belong to a single zone. */


The patch looks fine and does it job well. Could you resend it as a complete
patch with commit message and signed-off-by/reported-by lines? I will handle
merging it to mainline then.

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center





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