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

From: Ralph Campbell
Date: Mon Oct 25 2021 - 20:57:15 EST


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@xxxxxxxxxx>

You can add:
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

---
Documentation/vm/hmm.rst | 2 +-
arch/powerpc/kvm/book3s_hv_uvmem.c | 4 +-
drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 2 -
drivers/gpu/drm/nouveau/nouveau_dmem.c | 4 +-
include/linux/migrate.h | 1 -
lib/test_hmm.c | 5 +-
mm/migrate.c | 145 +++++------------------
7 files changed, 35 insertions(+), 128 deletions(-)

diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
index a14c2938e7af..f2a59ed82ed3 100644
--- a/Documentation/vm/hmm.rst
+++ b/Documentation/vm/hmm.rst
@@ -360,7 +360,7 @@ between device driver specific code and shared common code:
system memory page, locks the page with ``lock_page()``, and fills in the
``dst`` array entry with::
- dst[i] = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
+ dst[i] = migrate_pfn(page_to_pfn(dpage));
Now that the driver knows that this page is being migrated, it can
invalidate device private MMU mappings and copy device private memory
diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index a7061ee3b157..28c436df9935 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -560,7 +560,7 @@ static int __kvmppc_svm_page_out(struct vm_area_struct *vma,
gpa, 0, page_shift);
if (ret == U_SUCCESS)
- *mig.dst = migrate_pfn(pfn) | MIGRATE_PFN_LOCKED;
+ *mig.dst = migrate_pfn(pfn);
else {
unlock_page(dpage);
__free_page(dpage);
@@ -774,7 +774,7 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
}
}
- *mig.dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
+ *mig.dst = migrate_pfn(page_to_pfn(dpage));
migrate_vma_pages(&mig);
out_finalize:
migrate_vma_finalize(&mig);
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
index 4a16e3c257b9..41d9417f182b 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
@@ -300,7 +300,6 @@ svm_migrate_copy_to_vram(struct amdgpu_device *adev, struct svm_range *prange,
migrate->dst[i] = svm_migrate_addr_to_pfn(adev, dst[i]);
svm_migrate_get_vram_page(prange, migrate->dst[i]);
migrate->dst[i] = migrate_pfn(migrate->dst[i]);
- migrate->dst[i] |= MIGRATE_PFN_LOCKED;
src[i] = dma_map_page(dev, spage, 0, PAGE_SIZE,
DMA_TO_DEVICE);
r = dma_mapping_error(dev, src[i]);
@@ -580,7 +579,6 @@ svm_migrate_copy_to_ram(struct amdgpu_device *adev, struct svm_range *prange,
dst[i] >> PAGE_SHIFT, page_to_pfn(dpage));
migrate->dst[i] = migrate_pfn(page_to_pfn(dpage));
- migrate->dst[i] |= MIGRATE_PFN_LOCKED;
j++;
}
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index 92987daa5e17..3828aafd3ac4 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -166,7 +166,7 @@ static vm_fault_t nouveau_dmem_fault_copy_one(struct nouveau_drm *drm,
goto error_dma_unmap;
mutex_unlock(&svmm->mutex);
- args->dst[0] = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
+ args->dst[0] = migrate_pfn(page_to_pfn(dpage));
return 0;
error_dma_unmap:
@@ -602,7 +602,7 @@ static unsigned long nouveau_dmem_migrate_copy_one(struct nouveau_drm *drm,
((paddr >> PAGE_SHIFT) << NVIF_VMM_PFNMAP_V0_ADDR_SHIFT);
if (src & MIGRATE_PFN_WRITE)
*pfn |= NVIF_VMM_PFNMAP_V0_W;
- return migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
+ return migrate_pfn(page_to_pfn(dpage));
out_dma_unmap:
dma_unmap_page(dev, *dma_addr, PAGE_SIZE, DMA_BIDIRECTIONAL);
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index c8077e936691..479b861ae490 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -119,7 +119,6 @@ static inline int migrate_misplaced_page(struct page *page,
*/
#define MIGRATE_PFN_VALID (1UL << 0)
#define MIGRATE_PFN_MIGRATE (1UL << 1)
-#define MIGRATE_PFN_LOCKED (1UL << 2)
#define MIGRATE_PFN_WRITE (1UL << 3)
#define MIGRATE_PFN_SHIFT 6
diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index c259842f6d44..e2ce8f9b7605 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -613,8 +613,7 @@ static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
*/
rpage->zone_device_data = dmirror;
- *dst = migrate_pfn(page_to_pfn(dpage)) |
- MIGRATE_PFN_LOCKED;
+ *dst = migrate_pfn(page_to_pfn(dpage));
if ((*src & MIGRATE_PFN_WRITE) ||
(!spage && args->vma->vm_flags & VM_WRITE))
*dst |= MIGRATE_PFN_WRITE;
@@ -1137,7 +1136,7 @@ static vm_fault_t dmirror_devmem_fault_alloc_and_copy(struct migrate_vma *args,
lock_page(dpage);
xa_erase(&dmirror->pt, addr >> PAGE_SHIFT);
copy_highpage(dpage, spage);
- *dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
+ *dst = migrate_pfn(page_to_pfn(dpage));
if (*src & MIGRATE_PFN_WRITE)
*dst |= MIGRATE_PFN_WRITE;
}
diff --git a/mm/migrate.c b/mm/migrate.c
index a6a7743ee98f..915e969811d0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2369,7 +2369,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
* can't be dropped from it).
*/
get_page(page);
- migrate->cpages++;

Why not move the get_page() into the "if (trylock_page())" instead
of calling put_page() in the else case.

/*
* 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.

/* 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);