Re: [Question] Missing data after DMA read transfer - mm issue with transparent huge page?

[Kirill A. Shutemov]

On Tue, May 03, 2016 at 12:11:54PM +0200, Jerome Glisse wrote:
> On Mon, May 02, 2016 at 09:04:02PM -0700, Hugh Dickins wrote:
> > On Fri, 29 Apr 2016, Nicolas Morey Chaisemartin wrote:
> > 
> > > Hi everyone,
> > > 
> > > This is a repost from a different address as it seems the previous one ended in Gmail junk due to a domain error..
> > 
> > linux-kernel is a very high volume list which few are reading:
> > that also will account for your lack of response so far
> > (apart from the indefatigable Alan).
> > 
> > I've added linux-mm, and some people from another thread regarding
> > THP and get_user_pages() pins which has been discussed in recent days.
> > 
> > Make no mistake, the issue you're raising here is definitely not the
> > same as that one (which is specifically about the new THP refcounting
> > in v4.5+, whereas you're reporting a problem you've seen in both a
> > v3.10-based kernel and in v4.5).  But I think their heads are in
> > gear, much more so than mine, and likely to spot something.
> > 
> > > I added more info found while blindly debugging the issue.
> > > 
> > > Short version:
> > > I'm having an issue with direct DMA transfer from a device to host memory.
> > > It seems some of the data is not transferring to the appropriate page.
> > > 
> > > Some more details:
> > > I'm debugging a home made PCI driver for our board (Kalray), attached to a x86_64 host running centos7 (3.10.0-327.el7.x86_64)
> > > 
> > > In the current case, a userland application transfers back and forth data through read/write operations on a file.
> > > On the kernel side, it triggers DMA transfers through the PCI to/from our board memory.
> > > 
> > > We followed what pretty much all docs said about direct I/O to user buffers:
> > > 
> > > 1) get_user_pages() (in the current case, it's at most 16 pages at once)
> > > 2) convert to a scatterlist
> > > 3) pci_map_sg
> > > 4) eventually coalesce sg (Intel IOMMU is enabled, so it's usually possible)
> > > 4) A lot of DMA engine handling code, using the dmaengine layer and virt-dma
> > > 5) wait for transfer complete, in the mean time, go back to (1) to schedule more work, if any
> > > 6) pci_unmap_sg
> > > 7) for read (card2host) transfer, set_page_dirty_lock
> > > 8) page_cache_release
> > > 
> > > In 99,9999% it works perfectly.
> > > However, I have one userland application where a few pages are not written by a read (card2host) transfer.
> > > The buffer is memset them to a different value so I can check that nothing has overwritten them.
> > > 
> > > I know (PCI protocol analyser) that the data left our board for the "right" address (the one set in the sg by pci_map_sg).
> > > I tried reading the data between the pci_unmap_sg and the set_page_dirty, using
> > >         uint32_t *addr = page_address(trans->pages[0]);
> > >         dev_warn(&pdata->pdev->dev, "val = %x\n", *addr);
> > > and it has the expected value.
> > > But if I try to copy_from_user (using the address coming from userland, the one passed to get_user_pages), the data has not been written and I see the memset value.
> > > 
> > > New infos:
> > > 
> > > The issue happens with IOMMU on or off.
> > > I compiled a kernel with DMA_API_DEBUG enabled and got no warnings or errors.
> > > 
> > > I digged a little bit deeper with my very small understanding of linux mm and I discovered that:
> > >  * we are using transparent huge pages
> > >  * the page 'not transferred' are the last few of a huge page
> > > More precisely:
> > > - We have several transfer in flight from the same user buffer
> > > - Each transfer is 16 pages long
> > > - At one point in time, we start transferring from another huge page (transfers are still in flight from the previous one)
> > > - When a transfer from the previous huge page completes, I dumped at the mapcount of the pages from the previous transfers,
> > >   they are all to 0. The pages are still mapped to dma at this point.
> > > - A get_user_page to the address of the completed transfer returns return a different struct page * then the on I had.
> > > But this is before I have unmapped/put_page them back. From my understanding this should not have happened.
> > > 
> > > I tried the same code with a kernel 4.5 and encountered the same issue
> > > 
> > > Disabling transparent huge pages makes the issue disapear
> > > 
> > > Thanks in advance
> > 
> > It does look to me as if pages are being migrated, despite being pinned
> > by get_user_pages(): and that would be wrong.  Originally I intended
> > to suggest that THP is probably merely the cause of compaction, with
> > compaction causing the page migration.  But you posted very interesting
> > details in an earlier mail on 27th April from <nmorey@xxxxxxxxx>:
> > 
> > > I ran some more tests:
> > > 
> > > * Test is OK if transparent huge tlb are disabled
> > > 
> > > * For all the page where data are not transfered, and only those pages, a call to get_user_page(user vaddr) just before dma_unmap_sg returns a different page from the original one.
> > > [436477.927279] mppa 0000:03:00.0: org_page= ffffea0009f60080 cur page = ffffea00074e0080
> > > [436477.927298] page:ffffea0009f60080 count:0 mapcount:1 mapping:          (null) index:0x2
> > > [436477.927314] page flags: 0x2fffff00008000(tail)
> > > [436477.927354] page dumped because: org_page
> > > [436477.927369] page:ffffea00074e0080 count:0 mapcount:1 mapping:          (null) index:0x2
> > > [436477.927382] page flags: 0x2fffff00008000(tail)
> > > [436477.927421] page dumped because: cur_page
> > > 
> > > I'm not sure what to make of this...
> > 
> > That (on the older kernel I think) seems clearly to show that a THP
> > itself has been migrated: which makes me suspect NUMA migration of
> > mispaced THPs - migrate_misplaced_transhuge_page().  I'd hoped to
> > find something obviously wrong there, but haven't quite managed
> > to bring my brain fully to bear on it, and hope the others Cc'ed
> > will do so more quickly (or spot the error of your ways instead).
> > 
> > I do find it suspect, how the migrate_page_copy() is done rather
> > early, while the old page is still mapped in the pagetable.  And
> > odd how it inserts the new pmd for a moment, before checking old
> > page_count and backing out.  But I don't see how either of those
> > would cause the trouble you see, where the migration goes ahead.
> 
> So i do not think there is a bug migrate_misplaced_transhuge_page()
> but i think something is wrong in it see attached patch. I still
> want to convince myself i am not missing anything before posting
> that one.
> 
> 
> Now about this bug, dumb question but do you do get_user_pages with
> write = 1 because if your device is writting to the page then you
> must set write to 1.
> 
> get_user_pages(vaddr, nrpages, 1, 0|1, pages, NULL|vmas);
> 
> 
> Cheers,
> Jérôme

> From 9ded2a5da75a5e736fb36a2c4e2511d9516ecc37 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?J=C3=A9r=C3=B4me=20Glisse?= <jglisse@xxxxxxxxxx>
> Date: Tue, 3 May 2016 11:53:24 +0200
> Subject: [PATCH] mm/numa/thp: fix assumptions of
>  migrate_misplaced_transhuge_page()
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Fix assumptions in migrate_misplaced_transhuge_page() which is only
> call by do_huge_pmd_numa_page() itself only call by __handle_mm_fault()
> for pmd with PROT_NONE. This means that if the pmd stays the same
> then there can be no concurrent get_user_pages / get_user_pages_fast
> (GUP/GUP_fast). More over because migrate_misplaced_transhuge_page()
> only do something is page is map once then there can be no GUP from
> a different process. Finaly, holding the pmd lock assure us that no
> other part of the kernel will take an extre reference on the page.
> 
> In the end this means that the failure code path should never be
> taken unless something is horribly wrong, so convert it to BUG_ON().
> 
> Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>

The logic looks valid to me:

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

> ---
>  mm/migrate.c | 31 +++++++++++++++++++++----------
>  1 file changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6c822a7..6315aac 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1757,6 +1757,14 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  	pmd_t orig_entry;
>  
>  	/*
> +	 * What we do here is only valid if pmd_protnone(entry) is true and it
> +	 * is map in only one vma numamigrate_isolate_page() takes care of that
> +	 * check.
> +	 */
> +	if (!pmd_protnone(entry))
> +		goto out_unlock;
> +
> +	/*
>  	 * Rate-limit the amount of data that is being migrated to a node.
>  	 * Optimal placement is no good if the memory bus is saturated and
>  	 * all the time is being spent migrating!
> @@ -1797,7 +1805,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
>  	ptl = pmd_lock(mm, pmd);
>  	if (unlikely(!pmd_same(*pmd, entry) || page_count(page) != 2)) {
> -fail_putback:
>  		spin_unlock(ptl);
>  		mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
>  
> @@ -1819,7 +1826,12 @@ fail_putback:
>  		goto out_unlock;
>  	}
>  
> -	orig_entry = *pmd;
> +	/*
> +	 * We are holding the lock so no one can set a new pmd and original pmd
> +	 * is PROT_NONE thus no one can get_user_pages or get_user_pages_fast
> +	 * (GUP or GUP_fast) from this point on we can not fail.
> +	 */
> +	orig_entry = entry;
>  	entry = mk_pmd(new_page, vma->vm_page_prot);
>  	entry = pmd_mkhuge(entry);
>  	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> @@ -1837,14 +1849,13 @@ fail_putback:
>  	set_pmd_at(mm, mmun_start, pmd, entry);
>  	update_mmu_cache_pmd(vma, address, &entry);
>  
> -	if (page_count(page) != 2) {
> -		set_pmd_at(mm, mmun_start, pmd, orig_entry);
> -		flush_pmd_tlb_range(vma, mmun_start, mmun_end);
> -		mmu_notifier_invalidate_range(mm, mmun_start, mmun_end);
> -		update_mmu_cache_pmd(vma, address, &entry);
> -		page_remove_rmap(new_page, true);
> -		goto fail_putback;
> -	}
> +	/* As said above no one can get reference on the old page nor through
> +	 * get_user_pages or get_user_pages_fast (GUP/GUP_fast) or through
> +	 * any other means. To get reference on huge page you need to hold
> +	 * pmd_lock and we are already holding that lock here and the page
> +	 * is only mapped once.
> +	 */
> +	BUG_ON(page_count(page) != 2);
>  
>  	mlock_migrate_page(new_page, page);
>  	page_remove_rmap(page, true);
> -- 
> 2.1.0
> 


-- 
 Kirill A. Shutemov