linux-kernel - RE: [HMM 12/15] mm/migrate: new memory migration helper for use with device memory v4

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Date:   Tue, 27 Jun 2017 00:07:42 +0000
From:   Evgeny Baskakov <ebaskakov@...dia.com>
To:     Jérôme Glisse <jglisse@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
CC:     John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: RE: [HMM 12/15] mm/migrate: new memory migration helper for use with
 device memory v4

On Monday, May 22, 2017 9:52 AM, Jérôme Glisse wrote:
[...]

+ * The alloc_and_copy() callback happens once all source pages have 
+been locked,
+ * unmapped and checked (checked whether pinned or not). All pages that 
+can be
+ * migrated will have an entry in the src array set with the pfn value 
+of the
+ * page and with the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set 
+(other
+ * flags might be set but should be ignored by the callback).
+ *
+ * The alloc_and_copy() callback can then allocate destination memory 
+and copy
+ * source memory to it for all those entries (ie with MIGRATE_PFN_VALID 
+and
+ * MIGRATE_PFN_MIGRATE flag set). Once these are allocated and copied, 
+the
+ * callback must update each corresponding entry in the dst array with 
+the pfn
+ * value of the destination page and with the MIGRATE_PFN_VALID and
+ * MIGRATE_PFN_LOCKED flags set (destination pages must have their 
+struct pages
+ * locked, via lock_page()).
+ *
+ * At this point the alloc_and_copy() callback is done and returns.
+ *
+ * Note that the callback does not have to migrate all the pages that 
+are
+ * marked with MIGRATE_PFN_MIGRATE flag in src array unless this is a 
+migration
+ * from device memory to system memory (ie the MIGRATE_PFN_DEVICE flag 
+is also
+ * set in the src array entry). If the device driver cannot migrate a 
+device
+ * page back to system memory, then it must set the corresponding dst 
+array
+ * entry to MIGRATE_PFN_ERROR. This will trigger a SIGBUS if CPU tries 
+to
+ * access any of the virtual addresses originally backed by this page. 
+Because
+ * a SIGBUS is such a severe result for the userspace process, the 
+device
+ * driver should avoid setting MIGRATE_PFN_ERROR unless it is really in 
+an
+ * unrecoverable state.
+ *
+ * THE alloc_and_copy() CALLBACK MUST NOT CHANGE ANY OF THE SRC ARRAY 
+ENTRIES
+ * OR BAD THINGS WILL HAPPEN !
+ *

Hi Jerome,

The documentation shown above doesn't tell what the alloc_and_copy callback should do for source pages that have not been allocated yet. Instead, it unconditionally suggests checking if the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flags are set.

Based on my testing and looking in the source code, I see that for such pages the respective 'src' PFN entries are always set to 0 without any flags.

The sample driver specifically handles that by checking if there's no page in the 'src' entry, and ignores any flags in such case:

	struct page *spage = migrate_pfn_to_page(*src_pfns);
	...
	if (spage && !(*src_pfns & MIGRATE_PFN_MIGRATE))
		continue;

	if (spage && (*src_pfns & MIGRATE_PFN_DEVICE)) {

I would like to suggest reflecting that in the documentation. Or, which would be more logical, migrate_vma could keep the zero in the PFN entries for not allocated pages, but set the MIGRATE_PFN_MIGRATE flag anyway.

Thanks!

Evgeny Baskakov
NVIDIA