Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages

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On 1/29/20 5:51 AM, Kirill A. Shutemov wrote:
On Tue, Jan 28, 2020 at 07:24:13PM -0800, John Hubbard wrote:
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 not the limitation, discussed in pin_user_pages.rst under

s/not/note/

Fixed, thanks!

...
+/**
+ * page_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.

I wounder if we should reverse the logic and name -- page_not_dma_pinned()
or something -- too emphasise that we can only know for sure when the page
is not pinned, but not necessary when it is.


This is an interesting point. I agree that it's worth maybe adding information
into the function name, but I'd like to keep the bool "positive", because there
will be a number of callers that ask "if it is possibly dma-pinned, then ...".
So combining that, how about this function name:

	page_maybe_dma_pinned()

, which I could live with and I think would be acceptable?

+ *
+ * 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_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
+	 * from page_dma_pinned().
+	 */
+	return ((unsigned int)page_ref_count(compound_head(page))) >=
+		GUP_PIN_COUNTING_BIAS;

Do you expect it too be called on tail pages?


Yes, we definitely cannot rule that out.



+}
+
  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
  #define SECTION_IN_PAGE_FLAGS
  #endif
diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 14d14beb1f7f..b9cbe553d1e7 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -102,6 +102,16 @@ static inline void page_ref_sub(struct page *page, int nr)
  		__page_ref_mod(page, -nr);
  }
+static inline int page_ref_sub_return(struct page *page, int nr)
+{
+	int ret = atomic_sub_return(nr, &page->_refcount);
+
+	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
+		__page_ref_mod(page, -nr);
+
+	return ret;
+}
+

I see opportunity to split the patch further.


ah, OK. I wasn't sure how far to go before I get tagged for "excessive
patch splitting"! haha. Anyway, are you suggesting to put the
page_ref_sub_return() routine into it's own patch?

Another thing to split out would be adding the flags to the remaining
functions, such as undo_dev_pagemap(). That burns quite a few lines of
diff. Anything else to split out?


  static inline void page_ref_inc(struct page *page)
  {
  	atomic_inc(&page->_refcount);
diff --git a/mm/gup.c b/mm/gup.c
index 9e117998274c..7a96490dcc54 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -44,6 +44,136 @@ 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);

Maybe overflow detection? At least under VM_BUG_ON()?


OK, yes I see now that there is no check to see if we're about to overflow
the refs, in this path. I'll add one.


...
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0a55dec68925..b1079aaa6f24 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -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)))

Too many parentheses.


OK, I'll remove at least one. :)



+		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;
+

Ditto.


ACK.

...
@@ -4965,6 +4958,12 @@ 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;
+

Ditto.

ACK.




thanks,
--
John Hubbard
NVIDIA




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