[HMM 11/16] mm/hmm/mirror: device page fault handler

Jérôme Glisse
2017-04-05T20:43:29Z

This handle page fault on behalf of device driver, unlike handle_mm_fault()
it does not trigger migration back to system memory for device memory.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
include/linux/hmm.h | 27 ++++++
mm/hmm.c | 243 +++++++++++++++++++++++++++++++++++++++++++++++++---
2 files changed, 256 insertions(+), 14 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index defa7cd..d267989 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -292,6 +292,33 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
unsigned long end,
hmm_pfn_t *pfns);
bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range);
+
+
+/*
+ * Fault memory on behalf of device driver. Unlike handle_mm_fault(), this will
+ * not migrate any device memory back to system memory. The hmm_pfn_t array will
+ * be updated with the fault result and current snapshot of the CPU page table
+ * for the range.
+ *
+ * The mmap_sem must be taken in read mode before entering and it might be
+ * dropped by the function if the block argument is false. In that case, the
+ * function returns -EAGAIN.
+ *
+ * Return value does not reflect if the fault was successful for every single
+ * address or not. Therefore, the caller must to inspect the hmm_pfn_t array to
+ * determine fault status for each address.
+ *
+ * Trying to fault inside an invalid vma will result in -EINVAL.
+ *
+ * See the function description in mm/hmm.c for further documentation.
+ */
+int hmm_vma_fault(struct vm_area_struct *vma,
+ struct hmm_range *range,
+ unsigned long start,
+ unsigned long end,
+ hmm_pfn_t *pfns,
+ bool write,
+ bool block);
#endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */

diff --git a/mm/hmm.c b/mm/hmm.c
index 4828b97..be88807 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -235,6 +235,36 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
}
EXPORT_SYMBOL(hmm_mirror_unregister);

+struct hmm_vma_walk {
+ struct hmm_range *range;
+ unsigned long last;
+ bool fault;
+ bool block;
+ bool write;
+};
+
+static int hmm_vma_do_fault(struct mm_walk *walk,
+ unsigned long addr,
+ hmm_pfn_t *pfn)
+{
+ unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_REMOTE;
+ struct hmm_vma_walk *hmm_vma_walk = walk->private;
+ struct vm_area_struct *vma = walk->vma;
+ int r;
+
+ flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY;
+ flags |= hmm_vma_walk->write ? FAULT_FLAG_WRITE : 0;
+ r = handle_mm_fault(vma, addr, flags);
+ if (r & VM_FAULT_RETRY)
+ return -EBUSY;
+ if (r & VM_FAULT_ERROR) {
+ *pfn = HMM_PFN_ERROR;
+ return -EFAULT;
+ }
+
+ return -EAGAIN;
+}
+
static void hmm_pfns_special(hmm_pfn_t *pfns,
unsigned long addr,
unsigned long end)
@@ -243,34 +273,62 @@ static void hmm_pfns_special(hmm_pfn_t *pfns,
*pfns = HMM_PFN_SPECIAL;
}

+static void hmm_pfns_clear(hmm_pfn_t *pfns,
+ unsigned long addr,
+ unsigned long end)
+{
+ for (; addr < end; addr += PAGE_SIZE, pfns++)
+ *pfns = 0;
+}
+
static int hmm_vma_walk_hole(unsigned long addr,
unsigned long end,
struct mm_walk *walk)
{
- struct hmm_range *range = walk->private;
+ struct hmm_vma_walk *hmm_vma_walk = walk->private;
+ struct hmm_range *range = hmm_vma_walk->range;
hmm_pfn_t *pfns = range->pfns;
unsigned long i;

+ hmm_vma_walk->last = addr;
i = (addr - range->start) >> PAGE_SHIFT;
- for (; addr < end; addr += PAGE_SIZE, i++)
+ for (; addr < end; addr += PAGE_SIZE, i++) {
pfns[i] = HMM_PFN_EMPTY;
+ if (hmm_vma_walk->fault) {
+ int ret;

- return 0;
+ ret = hmm_vma_do_fault(walk, addr, &pfns[i]);
+ if (ret != -EAGAIN)
+ return ret;
+ }
+ }
+
+ return hmm_vma_walk->fault ? -EAGAIN : 0;
}

static int hmm_vma_walk_clear(unsigned long addr,
unsigned long end,
struct mm_walk *walk)
{
- struct hmm_range *range = walk->private;
+ struct hmm_vma_walk *hmm_vma_walk = walk->private;
+ struct hmm_range *range = hmm_vma_walk->range;
hmm_pfn_t *pfns = range->pfns;
unsigned long i;

+ hmm_vma_walk->last = addr;
i = (addr - range->start) >> PAGE_SHIFT;
- for (; addr < end; addr += PAGE_SIZE, i++)
+ for (; addr < end; addr += PAGE_SIZE, i++) {
pfns[i] = 0;
+ if (hmm_vma_walk->fault) {
+ int ret;

- return 0;
+ ret = hmm_vma_do_fault(walk, addr, &pfns[i]);
+ if (ret != -EAGAIN)
+ return ret;
+ }
+ }
+
+ return hmm_vma_walk->fault ? -EAGAIN : 0;
}

static int hmm_vma_walk_pmd(pmd_t *pmdp,
@@ -278,15 +336,18 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
unsigned long end,
struct mm_walk *walk)
{
- struct hmm_range *range = walk->private;
+ struct hmm_vma_walk *hmm_vma_walk = walk->private;
+ struct hmm_range *range = hmm_vma_walk->range;
struct vm_area_struct *vma = walk->vma;
hmm_pfn_t *pfns = range->pfns;
unsigned long addr = start, i;
+ bool write_fault;
hmm_pfn_t flag;
pte_t *ptep;

i = (addr - range->start) >> PAGE_SHIFT;
flag = vma->vm_flags & VM_READ ? HMM_PFN_READ : 0;
+ write_fault = hmm_vma_walk->fault & hmm_vma_walk->write;

if (pmd_none(*pmdp) || pmd_trans_unstable(pmdp)) {
pmd_t pmd;
@@ -302,6 +363,9 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
if (pmd_trans_huge(pmd) || pmd_devmap(pmd)) {
unsigned long pfn = pmd_pfn(pmd) + pte_index(addr);

+ if (write_fault && !pmd_write(pmd))
+ return hmm_vma_walk_clear(start, end, walk);
+
flag |= pmd_write(pmd) ? HMM_PFN_WRITE : 0;
for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
pfns[i] = hmm_pfn_t_from_pfn(pfn) | flag;
@@ -325,14 +389,19 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,

if (pte_none(pte)) {
pfns[i] = HMM_PFN_EMPTY;
+ if (hmm_vma_walk->fault)
+ goto fault;
continue;
}

if (!pte_present(pte)) {
swp_entry_t entry;

- if (!non_swap_entry(entry))
+ if (!non_swap_entry(entry)) {
+ if (hmm_vma_walk->fault)
+ goto fault;
continue;
+ }
entry = pte_to_swp_entry(pte);

/*
@@ -341,18 +410,38 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
*/
if (is_device_entry(entry)) {
pfns[i] = hmm_pfn_t_from_pfn(swp_offset(entry));
- if (is_write_device_entry(entry))
+ if (is_write_device_entry(entry)) {
pfns[i] |= HMM_PFN_WRITE;
+ } else if (write_fault)
+ goto fault;
pfns[i] |= HMM_PFN_DEVICE_UNADDRESSABLE;
pfns[i] |= flag;
- } else if (!is_migration_entry(entry)) {
+ } else if (is_migration_entry(entry)) {
+ if (hmm_vma_walk->fault) {
+ pte_unmap(ptep - 1);
+ hmm_vma_walk->last = addr;
+ migration_entry_wait(vma->vm_mm,
+ pmdp, addr);
+ return -EAGAIN;
+ }
+ continue;
+ } else {
+ /* Report error for everything else */
pfns[i] = HMM_PFN_ERROR;
}
continue;
}

+ if (write_fault && !pte_write(pte))
+ goto fault;
pfns[i] = hmm_pfn_t_from_pfn(pte_pfn(pte)) | flag;
pfns[i] |= pte_write(pte) ? HMM_PFN_WRITE : 0;
+ continue;
+
+fault:
+ pte_unmap(ptep - 1);
+ /* Fault all pages in range */
+ return hmm_vma_walk_clear(start, end, walk);
}
pte_unmap(ptep - 1);

@@ -385,6 +474,7 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
unsigned long end,
hmm_pfn_t *pfns)
{
+ struct hmm_vma_walk hmm_vma_walk;
struct mm_walk mm_walk;
struct hmm *hmm;

@@ -416,9 +506,12 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
list_add_rcu(&range->list, &hmm->ranges);
spin_unlock(&hmm->lock);

+ hmm_vma_walk.fault = false;
+ hmm_vma_walk.range = range;
+ mm_walk.private = &hmm_vma_walk;
+
mm_walk.vma = vma;
mm_walk.mm = vma->vm_mm;
- mm_walk.private = range;
mm_walk.pte_entry = NULL;
mm_walk.test_walk = NULL;
mm_walk.hugetlb_entry = NULL;
@@ -426,7 +519,6 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
mm_walk.pte_hole = hmm_vma_walk_hole;

walk_page_range(start, end, &mm_walk);
-
return 0;
}
EXPORT_SYMBOL(hmm_vma_get_pfns);
@@ -453,7 +545,7 @@ EXPORT_SYMBOL(hmm_vma_get_pfns);
*
* There are two ways to use this :
* again:
- * hmm_vma_get_pfns(vma, range, start, end, pfns);
+ * hmm_vma_get_pfns(vma, range, start, end, pfns); or hmm_vma_fault(...);
* trans = device_build_page_table_update_transaction(pfns);
* device_page_table_lock();
* if (!hmm_vma_range_done(vma, range)) {
@@ -464,7 +556,7 @@ EXPORT_SYMBOL(hmm_vma_get_pfns);
* device_page_table_unlock();
*
* Or:
- * hmm_vma_get_pfns(vma, range, start, end, pfns);
+ * hmm_vma_get_pfns(vma, range, start, end, pfns); or hmm_vma_fault(...);
* device_page_table_lock();
* hmm_vma_range_done(vma, range);
* device_update_page_table(pfns);
@@ -493,4 +585,127 @@ bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range)
return range->valid;
}
EXPORT_SYMBOL(hmm_vma_range_done);
+
+/*
+ * hmm_vma_fault() - try to fault some address in a virtual address range
+ * @vma: virtual memory area containing the virtual address range
+ * @range: use to track pfns array content validity
+ * @start: fault range virtual start address (inclusive)
+ * @end: fault range virtual end address (exclusive)
+ * @pfns: array of hmm_pfn_t, only entry with fault flag set will be faulted
+ * @write: is it a write fault
+ * @block: allow blocking on fault (if true it sleeps and do not drop mmap_sem)
+ * Returns: 0 success, error otherwise (-EAGAIN means mmap_sem have been drop)
+ *
+ * This is similar to a regular CPU page fault except that it will not trigger
+ * any memory migration if the memory being faulted is not accessible by CPUs.
+ *
+ * On error, for one virtual address in the range, the function will set the
+ * hmm_pfn_t error flag for the corresponding pfn entry.
+ *
+ * Expected use pattern:
+ * retry:
+ * down_read(&mm->mmap_sem);
+ * // Find vma and address device wants to fault, initialize hmm_pfn_t
+ * // array accordingly
+ * ret = hmm_vma_fault(vma, start, end, pfns, allow_retry);
+ * switch (ret) {
+ * case -EAGAIN:
+ * hmm_vma_range_done(vma, range);
+ * // You might want to rate limit or yield to play nicely, you may
+ * // also commit any valid pfn in the array assuming that you are
+ * // getting true from hmm_vma_range_monitor_end()
+ * goto retry;
+ * case 0:
+ * break;
+ * default:
+ * // Handle error !
+ * up_read(&mm->mmap_sem)
+ * return;
+ * }
+ * // Take device driver lock that serialize device page table update
+ * driver_lock_device_page_table_update();
+ * hmm_vma_range_done(vma, range);
+ * // Commit pfns we got from hmm_vma_fault()
+ * driver_unlock_device_page_table_update();
+ * up_read(&mm->mmap_sem)
+ *
+ * YOU MUST CALL hmm_vma_range_done() AFTER THIS FUNCTION RETURN SUCCESS (0)
+ * BEFORE FREEING THE range struct OR YOU WILL HAVE SERIOUS MEMORY CORRUPTION !
+ *
+ * YOU HAVE BEEN WARNED !
+ */
+int hmm_vma_fault(struct vm_area_struct *vma,
+ struct hmm_range *range,
+ unsigned long start,
+ unsigned long end,
+ hmm_pfn_t *pfns,
+ bool write,
+ bool block)
+{
+ struct hmm_vma_walk hmm_vma_walk;
+ struct mm_walk mm_walk;
+ struct hmm *hmm;
+ int ret;
+
+ /* Sanity check, this really should not happen ! */
+ if (start < vma->vm_start || start >= vma->vm_end)
+ return -EINVAL;
+ if (end < vma->vm_start || end > vma->vm_end)
+ return -EINVAL;
+
+ hmm = hmm_register(vma->vm_mm);
+ if (!hmm) {
+ hmm_pfns_clear(pfns, start, end);
+ return -ENOMEM;
+ }
+ /* Caller must have registered a mirror using hmm_mirror_register() */
+ if (!hmm->mmu_notifier.ops)
+ return -EINVAL;
+
+ /* Initialize range to track CPU page table update */
+ range->start = start;
+ range->pfns = pfns;
+ range->end = end;
+ spin_lock(&hmm->lock);
+ range->valid = true;
+ list_add_rcu(&range->list, &hmm->ranges);
+ spin_unlock(&hmm->lock);
+
+ /* FIXME support hugetlb fs */
+ if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL)) {
+ hmm_pfns_special(pfns, start, end);
+ return 0;
+ }
+
+ hmm_vma_walk.fault = true;
+ hmm_vma_walk.write = write;
+ hmm_vma_walk.block = block;
+ hmm_vma_walk.range = range;
+ mm_walk.private = &hmm_vma_walk;
+ hmm_vma_walk.last = range->start;
+
+ mm_walk.vma = vma;
+ mm_walk.mm = vma->vm_mm;
+ mm_walk.pte_entry = NULL;
+ mm_walk.test_walk = NULL;
+ mm_walk.hugetlb_entry = NULL;
+ mm_walk.pmd_entry = hmm_vma_walk_pmd;
+ mm_walk.pte_hole = hmm_vma_walk_hole;
+
+ do {
+ ret = walk_page_range(start, end, &mm_walk);
+ start = hmm_vma_walk.last;
+ } while (ret == -EAGAIN);
+
+ if (ret) {
+ unsigned long i;
+
+ i = (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
+ hmm_pfns_clear(&pfns[i], hmm_vma_walk.last, end);
+ hmm_vma_range_done(vma, range);
+ }
+ return ret;
+}
+EXPORT_SYMBOL(hmm_vma_fault);
#endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
--
2.9.3