@@ -52,8 +52,22 @@ Which flags are set by each wrapper
For these pin_user_pages*() functions, FOLL_PIN is OR'd in with whatever gup
flags the caller provides. The caller is required to pass in a non-null struct
-pages* array, and the function then pin pages by incrementing each by a special
-value. For now, that value is +1, just like get_user_pages*().::
+pages* array, and the function then pins pages by incrementing each by a special
+value: GUP_PIN_COUNTING_BIAS.
+
+For huge pages (and in fact, any compound page of more than 2 pages), the
+GUP_PIN_COUNTING_BIAS scheme is not used. Instead, an exact form of pin counting
+is achieved, by using the 3rd struct page in the compound page. A new struct
+page field, hpage_pinned_refcount, has been added in order to support this.
+
+This approach for compound pages avoids the counting upper limit problems that
+are discussed below. Those limitations would have been aggravated severely by
+huge pages, because each tail page adds a refcount to the head page. And in
+fact, testing revealed that, without a separate hpage_pinned_refcount field,
+page overflows were seen in some huge page stress tests.
+
+This also means that huge pages and compound pages (of order > 1) do not suffer
+from the false positives problem that is mentioned below.::
Function
--------
@@ -99,27 +113,6 @@ pages:
This also leads to limitations: there are only 31-10==21 bits available for a
counter that increments 10 bits at a time.
-TODO: for 1GB and larger huge pages, this is cutting it close. That's because
-when pin_user_pages() follows such pages, it increments the head page by "1"
-(where "1" used to mean "+1" for get_user_pages(), but now means "+1024" for
-pin_user_pages()) for each tail page. So if you have a 1GB huge page:
-
-* There are 256K (18 bits) worth of 4 KB tail pages.
-* There are 21 bits available to count up via GUP_PIN_COUNTING_BIAS (that is,
- 10 bits at a time)
-* There are 21 - 18 == 3 bits available to count. Except that there aren't,
- because you need to allow for a few normal get_page() calls on the head page,
- as well. Fortunately, the approach of using addition, rather than "hard"
- bitfields, within page->_refcount, allows for sharing these bits gracefully.
- But we're still looking at about 8 references.
-
-This, however, is a missing feature more than anything else, because it's easily
-solved by addressing an obvious inefficiency in the original get_user_pages()
-approach of retrieving pages: stop treating all the pages as if they were
-PAGE_SIZE. Retrieve huge pages as huge pages. The callers need to be aware of
-this, so some work is required. Once that's in place, this limitation mostly
-disappears from view, because there will be ample refcounting range available.
-
* Callers must specifically request "dma-pinned tracking of pages". In other
words, just calling get_user_pages() will not suffice; a new set of functions,
pin_user_page() and related, must be used.
@@ -228,5 +221,6 @@ References
* `Some slow progress on get_user_pages() (Apr 2, 2019) <https://lwn.net/Articles/784574/>`_
* `DMA and get_user_pages() (LPC: Dec 12, 2018) <https://lwn.net/Articles/774411/>`_
* `The trouble with get_user_pages() (Apr 30, 2018) <https://lwn.net/Articles/753027/>`_
+* `LWN kernel index: get_user_pages() <https://lwn.net/Kernel/Index/#Memory_management-get_user_pages>`_
John Hubbard, October, 2019
@@ -770,6 +770,24 @@ static inline unsigned int compound_orde
return page[1].compound_order;
}
+static inline bool hpage_pincount_available(struct page *page)
+{
+ /*
+ * Can the page->hpage_pinned_refcount field be used? That field is in
+ * the 3rd page of the compound page, so the smallest (2-page) compound
+ * pages cannot support it.
+ */
+ page = compound_head(page);
+ return PageCompound(page) && compound_order(page) > 1;
+}
+
+static inline int compound_pincount(struct page *page)
+{
+ VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+ page = compound_head(page);
+ return atomic_read(compound_pincount_ptr(page));
+}
+
static inline void set_compound_order(struct page *page, unsigned int order)
{
page[1].compound_order = order;
@@ -1084,6 +1102,11 @@ void unpin_user_pages(struct page **page
* refcounts, and b) all the callers of this routine are expected to be able to
* deal gracefully with a false positive.
*
+ * For huge pages, the result will be exactly correct. That's because we have
+ * more tracking data available: the 3rd struct page in the compound page is
+ * used to track the pincount (instead using of the GUP_PIN_COUNTING_BIAS
+ * scheme).
+ *
* For more information, please see Documentation/vm/pin_user_pages.rst.
*
* @page: pointer to page to be queried.
@@ -1092,6 +1115,9 @@ void unpin_user_pages(struct page **page
*/
static inline bool page_maybe_dma_pinned(struct page *page)
{
+ if (hpage_pincount_available(page))
+ return compound_pincount(page) > 0;
+
/*
* page_ref_count() is signed. If that refcount overflows, then
* page_ref_count() returns a negative value, and callers will avoid
@@ -137,7 +137,7 @@ struct page {
};
struct { /* Second tail page of compound page */
unsigned long _compound_pad_1; /* compound_head */
- unsigned long _compound_pad_2;
+ atomic_t hpage_pinned_refcount;
/* For both global and memcg */
struct list_head deferred_list;
};
@@ -226,6 +226,11 @@ static inline atomic_t *compound_mapcoun
return &page[1].compound_mapcount;
}
+static inline atomic_t *compound_pincount_ptr(struct page *page)
+{
+ return &page[2].hpage_pinned_refcount;
+}
+
/*
* Used for sizing the vmemmap region on some architectures
*/
@@ -29,6 +29,22 @@ struct follow_page_context {
unsigned int page_mask;
};
+static void hpage_pincount_add(struct page *page, int refs)
+{
+ VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+ VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+ atomic_add(refs, compound_pincount_ptr(page));
+}
+
+static void hpage_pincount_sub(struct page *page, int refs)
+{
+ VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+ VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+ atomic_sub(refs, compound_pincount_ptr(page));
+}
+
/*
* Return the compound head page with ref appropriately incremented,
* or NULL if that failed.
@@ -70,8 +86,25 @@ static __maybe_unused struct page *try_g
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);
+ /*
+ * When pinning a compound page of order > 1 (which is what
+ * hpage_pincount_available() checks for), use an exact count to
+ * track it, via hpage_pincount_add/_sub().
+ *
+ * However, be sure to *also* increment the normal page refcount
+ * field at least once, so that the page really is pinned.
+ */
+ if (!hpage_pincount_available(page))
+ refs *= GUP_PIN_COUNTING_BIAS;
+
+ page = try_get_compound_head(page, refs);
+ if (!page)
+ return NULL;
+
+ if (hpage_pincount_available(page))
+ hpage_pincount_add(page, refs);
+
+ return page;
}
WARN_ON_ONCE(1);
@@ -106,12 +139,25 @@ bool __must_check try_grab_page(struct p
if (flags & FOLL_GET)
return try_get_page(page);
else if (flags & FOLL_PIN) {
+ int refs = 1;
+
page = compound_head(page);
if (WARN_ON_ONCE(page_ref_count(page) <= 0))
return false;
- page_ref_add(page, GUP_PIN_COUNTING_BIAS);
+ if (hpage_pincount_available(page))
+ hpage_pincount_add(page, 1);
+ else
+ refs = GUP_PIN_COUNTING_BIAS;
+
+ /*
+ * Similar to try_grab_compound_head(): even if using the
+ * hpage_pincount_add/_sub() routines, be sure to
+ * *also* increment the normal page refcount field at least
+ * once, so that the page really is pinned.
+ */
+ page_ref_add(page, refs);
}
return true;
@@ -120,12 +166,17 @@ bool __must_check try_grab_page(struct p
#ifdef CONFIG_DEV_PAGEMAP_OPS
static bool __unpin_devmap_managed_user_page(struct page *page)
{
- int count;
+ int count, refs = 1;
if (!page_is_devmap_managed(page))
return false;
- count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+ if (hpage_pincount_available(page))
+ hpage_pincount_sub(page, 1);
+ else
+ refs = GUP_PIN_COUNTING_BIAS;
+
+ count = page_ref_sub_return(page, refs);
/*
* devmap page refcounts are 1-based, rather than 0-based: if
@@ -157,6 +208,8 @@ static bool __unpin_devmap_managed_user_
*/
void unpin_user_page(struct page *page)
{
+ int refs = 1;
+
page = compound_head(page);
/*
@@ -168,7 +221,12 @@ void unpin_user_page(struct page *page)
if (__unpin_devmap_managed_user_page(page))
return;
- if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+ if (hpage_pincount_available(page))
+ hpage_pincount_sub(page, 1);
+ else
+ refs = GUP_PIN_COUNTING_BIAS;
+
+ if (page_ref_sub_and_test(page, refs))
__put_page(page);
}
EXPORT_SYMBOL(unpin_user_page);
@@ -1955,8 +2013,12 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
static void put_compound_head(struct page *page, int refs, unsigned int flags)
{
- if (flags & FOLL_PIN)
- refs *= GUP_PIN_COUNTING_BIAS;
+ if (flags & FOLL_PIN) {
+ if (hpage_pincount_available(page))
+ hpage_pincount_sub(page, refs);
+ else
+ refs *= GUP_PIN_COUNTING_BIAS;
+ }
VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
/*
@@ -1009,6 +1009,9 @@ static void destroy_compound_gigantic_pa
struct page *p = page + 1;
atomic_set(compound_mapcount_ptr(page), 0);
+ if (hpage_pincount_available(page))
+ atomic_set(compound_pincount_ptr(page), 0);
+
for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
clear_compound_head(p);
set_page_refcounted(p);
@@ -1287,6 +1290,9 @@ static void prep_compound_gigantic_page(
set_compound_head(p, page);
}
atomic_set(compound_mapcount_ptr(page), -1);
+
+ if (hpage_pincount_available(page))
+ atomic_set(compound_pincount_ptr(page), 0);
}
/*
@@ -688,6 +688,8 @@ void prep_compound_page(struct page *pag
set_compound_head(p, page);
}
atomic_set(compound_mapcount_ptr(page), -1);
+ if (hpage_pincount_available(page))
+ atomic_set(compound_pincount_ptr(page), 0);
}
#ifdef CONFIG_DEBUG_PAGEALLOC
@@ -1178,6 +1178,9 @@ void page_add_new_anon_rmap(struct page
VM_BUG_ON_PAGE(!PageTransHuge(page), page);
/* increment count (starts at -1) */
atomic_set(compound_mapcount_ptr(page), 0);
+ if (hpage_pincount_available(page))
+ atomic_set(compound_pincount_ptr(page), 0);
+
__inc_node_page_state(page, NR_ANON_THPS);
} else {
/* Anon THP always mapped first with PMD */
@@ -1974,6 +1977,9 @@ void hugepage_add_new_anon_rmap(struct p
{
BUG_ON(address < vma->vm_start || address >= vma->vm_end);
atomic_set(compound_mapcount_ptr(page), 0);
+ if (hpage_pincount_available(page))
+ atomic_set(compound_pincount_ptr(page), 0);
+
__page_set_anon_rmap(page, vma, address, 1);
}
#endif /* CONFIG_HUGETLB_PAGE */