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

Alistair Popple
2021-10-28T01:27:29Z

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);
> >
>