@@ -173,8 +173,8 @@ CASE 4: Pinning for struct page manipulation only
-------------------------------------------------
Here, normal GUP calls are sufficient, so neither flag needs to be set.
-page_dma_pinned(): the whole point of pinning
-=============================================
+page_maybe_dma_pinned(): the whole point of pinning
+===================================================
The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able
to query, "is this page DMA-pinned?" That allows code such as page_mkclean()
@@ -186,7 +186,7 @@ and debates (see the References at the end of this document). It's a TODO item
here: fill in the details once that's worked out. Meanwhile, it's safe to say
that having this available: ::
- static inline bool page_dma_pinned(struct page *page)
+ static inline bool page_maybe_dma_pinned(struct page *page)
...is a prerequisite to solving the long-running gup+DMA problem.
@@ -1001,6 +1001,8 @@ static inline void get_page(struct page *page)
page_ref_inc(page);
}
+bool __must_check try_grab_page(struct page *page, unsigned int flags);
+
static inline __must_check bool try_get_page(struct page *page)
{
page = compound_head(page);
@@ -1029,29 +1031,79 @@ static inline void put_page(struct page *page)
__put_page(page);
}
-/**
- * unpin_user_page() - release a gup-pinned page
- * @page: pointer to page to be released
+/*
+ * GUP_PIN_COUNTING_BIAS, and the associated functions that use it, overload
+ * the page's refcount so that two separate items are tracked: the original page
+ * reference count, and also a new count of how many pin_user_pages() calls were
+ * made against the page. ("gup-pinned" is another term for the latter).
+ *
+ * With this scheme, pin_user_pages() becomes special: such pages are marked as
+ * distinct from normal pages. As such, the unpin_user_page() call (and its
+ * variants) must be used in order to release gup-pinned pages.
+ *
+ * Choice of value:
+ *
+ * By making GUP_PIN_COUNTING_BIAS a power of two, debugging of page reference
+ * counts with respect to pin_user_pages() and unpin_user_page() becomes
+ * simpler, due to the fact that adding an even power of two to the page
+ * refcount has the effect of using only the upper N bits, for the code that
+ * counts up using the bias value. This means that the lower bits are left for
+ * the exclusive use of the original code that increments and decrements by one
+ * (or at least, by much smaller values than the bias value).
*
- * Pages that were pinned via pin_user_pages*() must be released via either
- * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
- * that eventually such pages can be separately tracked and uniquely handled. In
- * particular, interactions with RDMA and filesystems need special handling.
+ * Of course, once the lower bits overflow into the upper bits (and this is
+ * OK, because subtraction recovers the original values), then visual inspection
+ * no longer suffices to directly view the separate counts. However, for normal
+ * applications that don't have huge page reference counts, this won't be an
+ * issue.
*
- * unpin_user_page() and put_page() are not interchangeable, despite this early
- * implementation that makes them look the same. unpin_user_page() calls must
- * be perfectly matched up with pin*() calls.
+ * Locking: the lockless algorithm described in page_cache_get_speculative()
+ * and page_cache_gup_pin_speculative() provides safe operation for
+ * get_user_pages and page_mkclean and other calls that race to set up page
+ * table entries.
*/
-static inline void unpin_user_page(struct page *page)
-{
- put_page(page);
-}
+#define GUP_PIN_COUNTING_BIAS (1U << 10)
+void unpin_user_page(struct page *page);
void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
bool make_dirty);
-
void unpin_user_pages(struct page **pages, unsigned long npages);
+/**
+ * page_maybe_dma_pinned() - report if a page is pinned for DMA.
+ *
+ * This function checks if a page has been pinned via a call to
+ * pin_user_pages*().
+ *
+ * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
+ * because it means "definitely not pinned for DMA", but true means "probably
+ * pinned for DMA, but possibly a false positive due to having at least
+ * GUP_PIN_COUNTING_BIAS worth of normal page references".
+ *
+ * False positives are OK, because: a) it's unlikely for a page to get that many
+ * refcounts, and b) all the callers of this routine are expected to be able to
+ * deal gracefully with a false positive.
+ *
+ * For more information, please see Documentation/vm/pin_user_pages.rst.
+ *
+ * @page: pointer to page to be queried.
+ * @Return: True, if it is likely that the page has been "dma-pinned".
+ * False, if the page is definitely not dma-pinned.
+ */
+static inline bool page_maybe_dma_pinned(struct page *page)
+{
+ /*
+ * page_ref_count() is signed. If that refcount overflows, then
+ * page_ref_count() returns a negative value, and callers will avoid
+ * further incrementing the refcount.
+ *
+ * Here, for that overflow case, use the signed bit to count a little
+ * bit higher via unsigned math, and thus still get an accurate result.
+ */
+ return ((unsigned int)page_ref_count(compound_head(page))) >=
+ GUP_PIN_COUNTING_BIAS;
+}
+
#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
#define SECTION_IN_PAGE_FLAGS
#endif
@@ -44,6 +44,135 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
return head;
}
+/*
+ * try_grab_compound_head() - attempt to elevate a page's refcount, by a
+ * flags-dependent amount.
+ *
+ * "grab" names in this file mean, "look at flags to decide whether to use
+ * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
+ *
+ * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
+ * same time. (That's true throughout the get_user_pages*() and
+ * pin_user_pages*() APIs.) Cases:
+ *
+ * FOLL_GET: page's refcount will be incremented by 1.
+ * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *
+ * Return: head page (with refcount appropriately incremented) for success, or
+ * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
+ * considered failure, and furthermore, a likely bug in the caller, so a warning
+ * is also emitted.
+ */
+static __maybe_unused struct page *try_grab_compound_head(struct page *page,
+ int refs,
+ unsigned int flags)
+{
+ if (flags & FOLL_GET)
+ return try_get_compound_head(page, refs);
+ else if (flags & FOLL_PIN) {
+ refs *= GUP_PIN_COUNTING_BIAS;
+ return try_get_compound_head(page, refs);
+ }
+
+ WARN_ON_ONCE(1);
+ return NULL;
+}
+
+/**
+ * try_grab_page() - elevate a page's refcount by a flag-dependent amount
+ *
+ * This might not do anything at all, depending on the flags argument.
+ *
+ * "grab" names in this file mean, "look at flags to decide whether to use
+ * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
+ *
+ * @page: pointer to page to be grabbed
+ * @flags: gup flags: these are the FOLL_* flag values.
+ *
+ * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
+ * time. Cases:
+ *
+ * FOLL_GET: page's refcount will be incremented by 1.
+ * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *
+ * Return: true for success, or if no action was required (if neither FOLL_PIN
+ * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
+ * FOLL_PIN was set, but the page could not be grabbed.
+ */
+bool __must_check try_grab_page(struct page *page, unsigned int flags)
+{
+ WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
+
+ if (flags & FOLL_GET)
+ return try_get_page(page);
+ else if (flags & FOLL_PIN) {
+ page = compound_head(page);
+
+ if (WARN_ON_ONCE(page_ref_count(page) <= 0))
+ return false;
+
+ page_ref_add(page, GUP_PIN_COUNTING_BIAS);
+ }
+
+ return true;
+}
+
+#ifdef CONFIG_DEV_PAGEMAP_OPS
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+ int count;
+
+ if (!page_is_devmap_managed(page))
+ return false;
+
+ count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+
+ /*
+ * devmap page refcounts are 1-based, rather than 0-based: if
+ * refcount is 1, then the page is free and the refcount is
+ * stable because nobody holds a reference on the page.
+ */
+ if (count == 1)
+ free_devmap_managed_page(page);
+ else if (!count)
+ __put_page(page);
+
+ return true;
+}
+#else
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+ return false;
+}
+#endif /* CONFIG_DEV_PAGEMAP_OPS */
+
+/**
+ * unpin_user_page() - release a dma-pinned page
+ * @page: pointer to page to be released
+ *
+ * Pages that were pinned via pin_user_pages*() must be released via either
+ * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
+ * that such pages can be separately tracked and uniquely handled. In
+ * particular, interactions with RDMA and filesystems need special handling.
+ */
+void unpin_user_page(struct page *page)
+{
+ page = compound_head(page);
+
+ /*
+ * For devmap managed pages we need to catch refcount transition from
+ * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
+ * page is free and we need to inform the device driver through
+ * callback. See include/linux/memremap.h and HMM for details.
+ */
+ if (__unpin_devmap_managed_user_page(page))
+ return;
+
+ if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+ __put_page(page);
+}
+EXPORT_SYMBOL(unpin_user_page);
+
/**
* unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
* @pages: array of pages to be maybe marked dirty, and definitely released.
@@ -230,10 +359,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
}
page = vm_normal_page(vma, address, pte);
- if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
+ if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
/*
- * Only return device mapping pages in the FOLL_GET case since
- * they are only valid while holding the pgmap reference.
+ * Only return device mapping pages in the FOLL_GET or FOLL_PIN
+ * case since they are only valid while holding the pgmap
+ * reference.
*/
*pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);
if (*pgmap)
@@ -271,11 +401,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
goto retry;
}
- if (flags & FOLL_GET) {
- if (unlikely(!try_get_page(page))) {
- page = ERR_PTR(-ENOMEM);
- goto out;
- }
+ /* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
+ if (unlikely(!try_grab_page(page, flags))) {
+ page = ERR_PTR(-ENOMEM);
+ goto out;
}
if (flags & FOLL_TOUCH) {
if ((flags & FOLL_WRITE) &&
@@ -537,7 +666,7 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
/* make this handle hugepd */
page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
if (!IS_ERR(page)) {
- BUG_ON(flags & FOLL_GET);
+ WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
return page;
}
@@ -1675,6 +1804,15 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
{
return 0;
}
+
+static long __get_user_pages_remote(struct task_struct *tsk,
+ struct mm_struct *mm,
+ unsigned long start, unsigned long nr_pages,
+ unsigned int gup_flags, struct page **pages,
+ struct vm_area_struct **vmas, int *locked)
+{
+ return 0;
+}
#endif /* !CONFIG_MMU */
/*
@@ -1877,7 +2015,10 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
struct page *page = pages[--(*nr)];
ClearPageReferenced(page);
- put_page(page);
+ if (flags & FOLL_PIN)
+ unpin_user_page(page);
+ else
+ put_page(page);
}
}
@@ -1919,7 +2060,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
page = pte_page(pte);
- head = try_get_compound_head(page, 1);
+ head = try_grab_compound_head(page, 1, flags);
if (!head)
goto pte_unmap;
@@ -1980,7 +2121,10 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
}
SetPageReferenced(page);
pages[*nr] = page;
- get_page(page);
+ if (unlikely(!try_grab_page(page, flags))) {
+ undo_dev_pagemap(nr, nr_start, flags, pages);
+ return 0;
+ }
(*nr)++;
pfn++;
} while (addr += PAGE_SIZE, addr != end);
@@ -2056,6 +2200,9 @@ static int record_subpages(struct page *page, unsigned long addr,
static void put_compound_head(struct page *page, int refs, unsigned int flags)
{
+ if (flags & FOLL_PIN)
+ refs *= GUP_PIN_COUNTING_BIAS;
+
VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
/*
* Calling put_page() for each ref is unnecessarily slow. Only the last
@@ -2099,7 +2246,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
refs = record_subpages(page, addr, end, pages + *nr);
- head = try_get_compound_head(head, refs);
+ head = try_grab_compound_head(head, refs, flags);
if (!head)
return 0;
@@ -2159,7 +2306,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
refs = record_subpages(page, addr, end, pages + *nr);
- head = try_get_compound_head(pmd_page(orig), refs);
+ head = try_grab_compound_head(pmd_page(orig), refs, flags);
if (!head)
return 0;
@@ -2193,7 +2340,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
refs = record_subpages(page, addr, end, pages + *nr);
- head = try_get_compound_head(pud_page(orig), refs);
+ head = try_grab_compound_head(pud_page(orig), refs, flags);
if (!head)
return 0;
@@ -2222,7 +2369,7 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
refs = record_subpages(page, addr, end, pages + *nr);
- head = try_get_compound_head(pgd_page(orig), refs);
+ head = try_grab_compound_head(pgd_page(orig), refs, flags);
if (!head)
return 0;
@@ -2505,11 +2652,11 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
/**
* get_user_pages_fast() - pin user pages in memory
- * @start: starting user address
- * @nr_pages: number of pages from start to pin
- * @gup_flags: flags modifying pin behaviour
- * @pages: array that receives pointers to the pages pinned.
- * Should be at least nr_pages long.
+ * @start: starting user address
+ * @nr_pages: number of pages from start to pin
+ * @gup_flags: flags modifying pin behaviour
+ * @pages: array that receives pointers to the pages pinned.
+ * Should be at least nr_pages long.
*
* Attempt to pin user pages in memory without taking mm->mmap_sem.
* If not successful, it will fall back to taking the lock and
@@ -2543,9 +2690,12 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
/**
* pin_user_pages_fast() - pin user pages in memory without taking locks
*
- * For now, this is a placeholder function, until various call sites are
- * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
- * this is identical to get_user_pages_fast().
+ * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
+ * get_user_pages_fast() for documentation on the function arguments, because
+ * the arguments here are identical.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/vm/pin_user_pages.rst for further details.
*
* This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
* is NOT intended for Case 2 (RDMA: long-term pins).
@@ -2553,21 +2703,24 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
int pin_user_pages_fast(unsigned long start, int nr_pages,
unsigned int gup_flags, struct page **pages)
{
- /*
- * This is a placeholder, until the pin functionality is activated.
- * Until then, just behave like the corresponding get_user_pages*()
- * routine.
- */
- return get_user_pages_fast(start, nr_pages, gup_flags, pages);
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+ return -EINVAL;
+
+ gup_flags |= FOLL_PIN;
+ return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
}
EXPORT_SYMBOL_GPL(pin_user_pages_fast);
/**
* pin_user_pages_remote() - pin pages of a remote process (task != current)
*
- * For now, this is a placeholder function, until various call sites are
- * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
- * this is identical to get_user_pages_remote().
+ * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
+ * get_user_pages_remote() for documentation on the function arguments, because
+ * the arguments here are identical.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/vm/pin_user_pages.rst for details.
*
* This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
* is NOT intended for Case 2 (RDMA: long-term pins).
@@ -2577,22 +2730,24 @@ long pin_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas, int *locked)
{
- /*
- * This is a placeholder, until the pin functionality is activated.
- * Until then, just behave like the corresponding get_user_pages*()
- * routine.
- */
- return get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags, pages,
- vmas, locked);
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+ return -EINVAL;
+
+ gup_flags |= FOLL_PIN;
+ return __get_user_pages_remote(tsk, mm, start, nr_pages, gup_flags,
+ pages, vmas, locked);
}
EXPORT_SYMBOL(pin_user_pages_remote);
/**
* pin_user_pages() - pin user pages in memory for use by other devices
*
- * For now, this is a placeholder function, until various call sites are
- * converted to use the correct get_user_pages*() or pin_user_pages*() API. So,
- * this is identical to get_user_pages().
+ * Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
+ * FOLL_PIN is set.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/vm/pin_user_pages.rst for details.
*
* This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
* is NOT intended for Case 2 (RDMA: long-term pins).
@@ -2601,11 +2756,12 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas)
{
- /*
- * This is a placeholder, until the pin functionality is activated.
- * Until then, just behave like the corresponding get_user_pages*()
- * routine.
- */
- return get_user_pages(start, nr_pages, gup_flags, pages, vmas);
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+ return -EINVAL;
+
+ gup_flags |= FOLL_PIN;
+ return __gup_longterm_locked(current, current->mm, start, nr_pages,
+ pages, vmas, gup_flags);
}
EXPORT_SYMBOL(pin_user_pages);
@@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
*/
WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+ (FOLL_PIN | FOLL_GET)))
+ return NULL;
+
if (flags & FOLL_WRITE && !pmd_write(*pmd))
return NULL;
@@ -973,7 +978,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
* device mapped pages can only be returned if the
* caller will manage the page reference count.
*/
- if (!(flags & FOLL_GET))
+ if (!(flags & (FOLL_GET | FOLL_PIN)))
return ERR_PTR(-EEXIST);
pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
@@ -981,7 +986,8 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
if (!*pgmap)
return ERR_PTR(-EFAULT);
page = pfn_to_page(pfn);
- get_page(page);
+ if (!try_grab_page(page, flags))
+ page = ERR_PTR(-ENOMEM);
return page;
}
@@ -1101,6 +1107,11 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
if (flags & FOLL_WRITE && !pud_write(*pud))
return NULL;
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+ (FOLL_PIN | FOLL_GET)))
+ return NULL;
+
if (pud_present(*pud) && pud_devmap(*pud))
/* pass */;
else
@@ -1112,8 +1123,10 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
/*
* device mapped pages can only be returned if the
* caller will manage the page reference count.
+ *
+ * At least one of FOLL_GET | FOLL_PIN must be set, so assert that here:
*/
- if (!(flags & FOLL_GET))
+ if (!(flags & (FOLL_GET | FOLL_PIN)))
return ERR_PTR(-EEXIST);
pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
@@ -1121,7 +1134,8 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
if (!*pgmap)
return ERR_PTR(-EFAULT);
page = pfn_to_page(pfn);
- get_page(page);
+ if (!try_grab_page(page, flags))
+ page = ERR_PTR(-ENOMEM);
return page;
}
@@ -1497,8 +1511,13 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
page = pmd_page(*pmd);
VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+
+ if (!try_grab_page(page, flags))
+ return ERR_PTR(-ENOMEM);
+
if (flags & FOLL_TOUCH)
touch_pmd(vma, addr, pmd, flags);
+
if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
/*
* We don't mlock() pte-mapped THPs. This way we can avoid
@@ -1535,8 +1554,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
skip_mlock:
page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
- if (flags & FOLL_GET)
- get_page(page);
out:
return page;
@@ -4375,19 +4375,6 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
page = pte_page(huge_ptep_get(pte));
- /*
- * Instead of doing 'try_get_page()' below in the same_page
- * loop, just check the count once here.
- */
- if (unlikely(page_count(page) <= 0)) {
- if (pages) {
- spin_unlock(ptl);
- remainder = 0;
- err = -ENOMEM;
- break;
- }
- }
-
/*
* If subpage information not requested, update counters
* and skip the same_page loop below.
@@ -4405,7 +4392,22 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
same_page:
if (pages) {
pages[i] = mem_map_offset(page, pfn_offset);
- get_page(pages[i]);
+ /*
+ * try_grab_page() should always succeed here, because:
+ * a) we hold the ptl lock, and b) we've just checked
+ * that the huge page is present in the page tables. If
+ * the huge page is present, then the tail pages must
+ * also be present. The ptl prevents the head page and
+ * tail pages from being rearranged in any way. So this
+ * page must be available at this point, unless the page
+ * refcount overflowed:
+ */
+ if (WARN_ON_ONCE(!try_grab_page(pages[i], flags))) {
+ spin_unlock(ptl);
+ remainder = 0;
+ err = -ENOMEM;
+ break;
+ }
}
if (vmas)
@@ -4965,6 +4967,12 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
struct page *page = NULL;
spinlock_t *ptl;
pte_t pte;
+
+ /* FOLL_GET and FOLL_PIN are mutually exclusive. */
+ if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+ (FOLL_PIN | FOLL_GET)))
+ return NULL;
+
retry:
ptl = pmd_lockptr(mm, pmd);
spin_lock(ptl);
@@ -4977,8 +4985,18 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
pte = huge_ptep_get((pte_t *)pmd);
if (pte_present(pte)) {
page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
- if (flags & FOLL_GET)
- get_page(page);
+ /*
+ * try_grab_page() should always succeed here, because: a) we
+ * hold the pmd (ptl) lock, and b) we've just checked that the
+ * huge pmd (head) page is present in the page tables. The ptl
+ * prevents the head page and tail pages from being rearranged
+ * in any way. So this page must be available at this point,
+ * unless the page refcount overflowed:
+ */
+ if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
+ page = NULL;
+ goto out;
+ }
} else {
if (is_hugetlb_entry_migration(pte)) {
spin_unlock(ptl);
@@ -4999,7 +5017,7 @@ struct page * __weak
follow_huge_pud(struct mm_struct *mm, unsigned long address,
pud_t *pud, int flags)
{
- if (flags & FOLL_GET)
+ if (flags & (FOLL_GET | FOLL_PIN))
return NULL;
return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
@@ -5008,7 +5026,7 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address,
struct page * __weak
follow_huge_pgd(struct mm_struct *mm, unsigned long address, pgd_t *pgd, int flags)
{
- if (flags & FOLL_GET)
+ if (flags & (FOLL_GET | FOLL_PIN))
return NULL;
return pte_page(*(pte_t *)pgd) + ((address & ~PGDIR_MASK) >> PAGE_SHIFT);
Add tracking of pages that were pinned via FOLL_PIN. This tracking is implemented via overloading of page->_refcount: pins are added by adding GUP_PIN_COUNTING_BIAS (1024) to the refcount. This provides a fuzzy indication of pinning, and it can have false positives (and that's OK). Please see the pre-existing Documentation/core-api/pin_user_pages.rst for details. As mentioned in pin_user_pages.rst, callers who effectively set FOLL_PIN (typically via pin_user_pages*()) are required to ultimately free such pages via unpin_user_page(). Please also note the limitation, discussed in pin_user_pages.rst under the "TODO: for 1GB and larger huge pages" section. (That limitation will be removed in a following patch.) The effect of a FOLL_PIN flag is similar to that of FOLL_GET, and may be thought of as "FOLL_GET for DIO and/or RDMA use". Pages that have been pinned via FOLL_PIN are identifiable via a new function call: bool page_maybe_dma_pinned(struct page *page); What to do in response to encountering such a page, is left to later patchsets. There is discussion about this in [1], [2], [3], and [4]. This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask(). [1] Some slow progress on get_user_pages() (Apr 2, 2019): https://lwn.net/Articles/784574/ [2] DMA and get_user_pages() (LPC: Dec 12, 2018): https://lwn.net/Articles/774411/ [3] The trouble with get_user_pages() (Apr 30, 2018): https://lwn.net/Articles/753027/ [4] LWN kernel index: get_user_pages(): https://lwn.net/Kernel/Index/#Memory_management-get_user_pages Reviewed-by: Jan Kara <jack@suse.cz> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Suggested-by: Jan Kara <jack@suse.cz> Suggested-by: Jérôme Glisse <jglisse@redhat.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- Documentation/core-api/pin_user_pages.rst | 6 +- include/linux/mm.h | 82 +++++-- mm/gup.c | 254 +++++++++++++++++----- mm/huge_memory.c | 29 ++- mm/hugetlb.c | 54 +++-- 5 files changed, 334 insertions(+), 91 deletions(-)