Re: [PATCH] mm/migrate.c: Remove MIGRATE_PFN_LOCKED

On Tuesday, 26 October 2021 11:57:06 AM AEDT Ralph Campbell wrote:

On 10/24/21 21:16, Alistair Popple wrote:

...

MIGRATE_PFN_LOCKED is used to indicate to migrate_vma_prepare() that a source page was already locked during migrate_vma_collect(). If it wasn't then the a second attempt is made to lock the page. However if the first attempt failed it's unlikely a second attempt will succeed, and the retry adds complexity. So clean this up by removing the retry and MIGRATE_PFN_LOCKED flag.

Destination pages are also meant to have the MIGRATE_PFN_LOCKED flag set, but nothing actually checks that.

Signed-off-by: Alistair Popple apopple@nvidia.com

You can add: Reviewed-by: Ralph Campbell rcampbell@nvidia.com

Thanks!

...

/*
 * Optimize for the common case where page is only mapped once

@@ -2379,7 +2378,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, if (trylock_page(page)) { pte_t swp_pte;

• 	mpfn |= MIGRATE_PFN_LOCKED;
  

• 	migrate->cpages++;
  ptep_get_and_clear(mm, addr, ptep);
  

I was looking at try_to_migrate_one() and looking at the differences with the code here to insert the migration PTE and noticed that instead of ptet_get_and_clear() it has: pteval = ptep_clear_flush(vma, address, pvmw.pte);

/* Move the dirty bit to the page. Now the pte is gone. */ if (pte_dirty(pteval)) set_page_dirty(page); update_hiwater_rss(mm);

I know that is pre-existing, probably a separate patch if it is an issue.

I don't think it is an issue today because migrate_vma only supports private, non-shared pages. At some point though it may be extended to support file-backed pages and this would be easy to miss so will put a patch together.

...

	/* Setup special migration page table entry */

@@ -2413,6 +2412,9 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,

	if (pte_present(pte))
		unmapped++;

• } else {
  

• 	put_page(page);
  

• 	mpfn = 0;
  

  }

  next:

@@ -2517,15 +2519,17 @@ static bool migrate_vma_check_page(struct page *page) }

/*

• • migrate_vma_prepare() - lock pages and isolate them from the lru

• • migrate_vma_unmap() - replace page mapping with special migration pte entry
  • @migrate: migrate struct containing all migration information

• • This locks pages that have been collected by migrate_vma_collect(). Once each
• • page is locked it is isolated from the lru (for non-device pages). Finally,
• • the ref taken by migrate_vma_collect() is dropped, as locked pages cannot be
• • migrated by concurrent kernel threads.

• • Isolate pages from the LRU and replace mappings (CPU page table pte) with a
• • special migration pte entry and check if it has been pinned. Pinned pages are
• • restored because we cannot migrate them.
• • This is the last step before we call the device driver callback to allocate
• • destination memory and copy contents of original page over to new page.
  */

-static void migrate_vma_prepare(struct migrate_vma *migrate) +static void migrate_vma_unmap(struct migrate_vma *migrate) { const unsigned long npages = migrate->npages; const unsigned long start = migrate->start; @@ -2534,32 +2538,12 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)

lru_add_drain();

• for (i = 0; (i < npages) && migrate->cpages; i++) {

• for (i = 0; i < npages; i++) { struct page *page = migrate_pfn_to_page(migrate->src[i]);

• bool remap = true;
  
  

  if (!page) continue;

• if (!(migrate->src[i] & MIGRATE_PFN_LOCKED)) {
  

• 	/*
  

• 	 * Because we are migrating several pages there can be
  

• 	 * a deadlock between 2 concurrent migration where each
  

• 	 * are waiting on each other page lock.
  

• 	 *
  

• 	 * Make migrate_vma() a best effort thing and backoff
  

• 	 * for any page we can not lock right away.
  

• 	 */
  

• 	if (!trylock_page(page)) {
  

• 		migrate->src[i] = 0;
  

• 		migrate->cpages--;
  

• 		put_page(page);
  

• 		continue;
  

• 	}
  

• 	remap = false;
  

• 	migrate->src[i] |= MIGRATE_PFN_LOCKED;
  

• }
  

• /* ZONE_DEVICE pages are not on LRU */ if (!is_zone_device_page(page)) { if (!PageLRU(page) && allow_drain) {

@@ -2569,16 +2553,9 @@ static void migrate_vma_prepare(struct migrate_vma *migrate) }

	if (isolate_lru_page(page)) {

• 		if (remap) {
  

• 			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
  

• 			migrate->cpages--;
  

• 			restore++;
  

• 		} else {
  

• 			migrate->src[i] = 0;
  

• 			unlock_page(page);
  

• 			migrate->cpages--;
  

• 			put_page(page);
  

• 		}
  

• 		migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
  

• 		migrate->cpages--;
  

• 		restore++;
  	continue;
  }
  
  

@@ -2586,80 +2563,20 @@ static void migrate_vma_prepare(struct migrate_vma *migrate) put_page(page); }

• if (!migrate_vma_check_page(page)) {
  

• 	if (remap) {
  

• 		migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
  

• 		migrate->cpages--;
  

• 		restore++;
  

• 		if (!is_zone_device_page(page)) {
  

• 			get_page(page);
  

• 			putback_lru_page(page);
  

• 		}
  

• 	} else {
  

• 		migrate->src[i] = 0;
  

• 		unlock_page(page);
  

• 		migrate->cpages--;
  

• if (page_mapped(page))
  

• 	try_to_migrate(page, 0);
  
  

• 		if (!is_zone_device_page(page))
  

• 			putback_lru_page(page);
  

• 		else
  

• 			put_page(page);
  

• if (page_mapped(page) || !migrate_vma_check_page(page)) {
  

• 	if (!is_zone_device_page(page)) {
  

• 		get_page(page);
  

• 		putback_lru_page(page);
  }
  

• }
  

• }

• for (i = 0, addr = start; i < npages && restore; i++, addr += PAGE_SIZE) {

• struct page *page = migrate_pfn_to_page(migrate->src[i]);
  

• if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
  

• 	continue;
  
  

• remove_migration_pte(page, migrate->vma, addr, page);
  

• migrate->src[i] = 0;
  

• unlock_page(page);
  

• put_page(page);
  

• restore--;
  

• }

-}

-/*

• • migrate_vma_unmap() - replace page mapping with special migration pte entry
• • @migrate: migrate struct containing all migration information
• • Replace page mapping (CPU page table pte) with a special migration pte entry
• • and check again if it has been pinned. Pinned pages are restored because we
• • cannot migrate them.
• • This is the last step before we call the device driver callback to allocate
• • destination memory and copy contents of original page over to new page.
• */

-static void migrate_vma_unmap(struct migrate_vma *migrate) -{

• const unsigned long npages = migrate->npages;
• const unsigned long start = migrate->start;
• unsigned long addr, i, restore = 0;
• for (i = 0; i < npages; i++) {

• struct page *page = migrate_pfn_to_page(migrate->src[i]);
  

• if (!page || !(migrate->src[i] & MIGRATE_PFN_MIGRATE))
  

• 	migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
  

• 	migrate->cpages--;
  

• 	restore++;
  continue;
  

• if (page_mapped(page)) {
  

• 	try_to_migrate(page, 0);
  

• 	if (page_mapped(page))
  

• 		goto restore;
  

  }

• if (migrate_vma_check_page(page))
  

• 	continue;
  

-restore:

• migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
  

• migrate->cpages--;
  

• restore++;
  

  }

  for (addr = start, i = 0; i < npages && restore; addr += PAGE_SIZE, i++) {

@@ -2672,12 +2589,8 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)

migrate->src[i] = 0;
unlock_page(page);

• put_page(page);
  

  restore--;

• if (is_zone_device_page(page))
  

• 	put_page(page);
  

• else
  

• 	putback_lru_page(page);
  

  } }

@@ -2700,8 +2613,8 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)

• it for all those entries (ie with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE
• flag set). Once these are allocated and copied, the caller 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()).

• • page and with MIGRATE_PFN_VALID. Destination pages must be locked via
• • lock_page().
  • Note that the caller 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

@@ -2770,8 +2683,6 @@ int migrate_vma_setup(struct migrate_vma *args)

migrate_vma_collect(args);

• if (args->cpages)

• migrate_vma_prepare(args);
  

  if (args->cpages) migrate_vma_unmap(args);