Re: [RFC RESEND 00/16] Split IOMMU DMA mapping operation to two steps

[Leon Romanovsky <leon@kernel.org>]

On Tue, Mar 19, 2024 at 12:36:20PM -0300, Jason Gunthorpe wrote:
> On Sat, Mar 09, 2024 at 05:14:18PM +0100, Christoph Hellwig wrote:
> > On Fri, Mar 08, 2024 at 04:23:42PM -0400, Jason Gunthorpe wrote:
> > > > The DMA API callers really need to know what is P2P or not for
> > > > various reasons.  And they should generally have that information
> > > > available, either from pin_user_pages that needs to special case
> > > > it or from the in-kernel I/O submitter that build it from P2P and
> > > > normal memory.
> > > 
> > > I think that is a BIO thing. RDMA just calls with FOLL_PCI_P2PDMA and
> > > shoves the resulting page list into in a scattertable. It never checks
> > > if any returned page is P2P - it has no reason to care. dma_map_sg()
> > > does all the work.
> > 
> > Right now it does, but that's not really a good interface.  If we have
> > a pin_user_pages variant that only pins until the next relevant P2P
> > boundary and tells you about we can significantly simplify the overall
> > interface.
> 
> Sorry for the delay, I was away..

<...>

> Can we tweak what Leon has done to keep the hmm_range_fault support
> and non-uniformity for RDMA but add a uniformity optimized flow for
> BIO?

Something like this will do the trick.

From 45e739e7073fb04bc168624f77320130bb3f9267 Mon Sep 17 00:00:00 2001
Message-ID: <45e739e7073fb04bc168624f77320130bb3f9267.1710924764.git.leonro@nvidia.com>
From: Leon Romanovsky <leonro@nvidia.com>
Date: Mon, 18 Mar 2024 11:16:41 +0200
Subject: [PATCH] mm/gup: add strict interface to pin user pages according to
 FOLL flag

All pin_user_pages*() and get_user_pages*() callbacks allocate user
pages by partially taking into account their p2p vs. non-p2p properties.

In case, user sets FOLL_PCI_P2PDMA flag, the allocated pages will include
both p2p and "regular" pages, while if FOLL_PCI_P2PDMA flag is not provided,
only regular pages are returned.

In order to make sure that with FOLL_PCI_P2PDMA flag, only p2p pages are
returned, let's introduce new internal FOLL_STRICT flag and provide special
pin_user_pages_fast_strict() API call.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
 include/linux/mm.h |  3 +++
 mm/gup.c           | 36 +++++++++++++++++++++++++++++++++++-
 mm/internal.h      |  4 +++-
 3 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f5a97dec5169..910b65dde24a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2491,6 +2491,9 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages);
 void folio_add_pin(struct folio *folio);
 
+int pin_user_pages_fast_strict(unsigned long start, int nr_pages,
+			       unsigned int gup_flags, struct page **pages);
+
 int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc);
 int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
 			struct task_struct *task, bool bypass_rlim);
diff --git a/mm/gup.c b/mm/gup.c
index df83182ec72d..11b5c626a4ab 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -133,6 +133,10 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
 	if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
 		return NULL;
 
+	if (flags & FOLL_STRICT)
+		if (flags & FOLL_PCI_P2PDMA && !is_pci_p2pdma_page(page))
+			return NULL;
+
 	if (flags & FOLL_GET)
 		return try_get_folio(page, refs);
 
@@ -232,6 +236,10 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
 	if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
 		return -EREMOTEIO;
 
+	if (flags & FOLL_STRICT)
+		if (flags & FOLL_PCI_P2PDMA && !is_pci_p2pdma_page(page))
+			return -EREMOTEIO;
+
 	if (flags & FOLL_GET)
 		folio_ref_inc(folio);
 	else if (flags & FOLL_PIN) {
@@ -2243,6 +2251,8 @@ static bool is_valid_gup_args(struct page **pages, int *locked,
 	 * - FOLL_TOUCH/FOLL_PIN/FOLL_TRIED/FOLL_FAST_ONLY are internal only
 	 * - FOLL_REMOTE is internal only and used on follow_page()
 	 * - FOLL_UNLOCKABLE is internal only and used if locked is !NULL
+	 * - FOLL_STRICT is internal only and used to distinguish between p2p
+	 *   and "regular" pages.
 	 */
 	if (WARN_ON_ONCE(gup_flags & INTERNAL_GUP_FLAGS))
 		return false;
@@ -3187,7 +3197,8 @@ static int internal_get_user_pages_fast(unsigned long start,
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
 				       FOLL_FORCE | FOLL_PIN | FOLL_GET |
 				       FOLL_FAST_ONLY | FOLL_NOFAULT |
-				       FOLL_PCI_P2PDMA | FOLL_HONOR_NUMA_FAULT)))
+				       FOLL_PCI_P2PDMA | FOLL_HONOR_NUMA_FAULT |
+				       FOLL_STRICT)))
 		return -EINVAL;
 
 	if (gup_flags & FOLL_PIN)
@@ -3322,6 +3333,29 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
 }
 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
 
+/**
+ * pin_user_pages_fast_strict() - this is pin_user_pages_fast() variant, which
+ * makes sure that only pages with same properties are pinned.
+ *
+ * @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.
+ *
+ * Nearly the same as pin_user_pages_fastt(), except that FOLL_STRICT is set.
+ *
+ * FOLL_STRICT means that the pages are allocated with specific FOLL_* properties.
+ */
+int pin_user_pages_fast_strict(unsigned long start, int nr_pages,
+			       unsigned int gup_flags, struct page **pages)
+{
+	if (!is_valid_gup_args(pages, NULL, &gup_flags, FOLL_PIN | FOLL_STRICT))
+		return -EINVAL;
+	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fast_strict);
+
 /**
  * pin_user_pages_remote() - pin pages of a remote process
  *
diff --git a/mm/internal.h b/mm/internal.h
index f309a010d50f..7578837a0444 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1031,10 +1031,12 @@ enum {
 	FOLL_FAST_ONLY = 1 << 20,
 	/* allow unlocking the mmap lock */
 	FOLL_UNLOCKABLE = 1 << 21,
+	/* don't mix pages with different properties, e.g. p2p with "regular" ones */
+	FOLL_STRICT = 1 << 22,
 };
 
 #define INTERNAL_GUP_FLAGS (FOLL_TOUCH | FOLL_TRIED | FOLL_REMOTE | FOLL_PIN | \
-			    FOLL_FAST_ONLY | FOLL_UNLOCKABLE)
+			    FOLL_FAST_ONLY | FOLL_UNLOCKABLE | FOLL_STRICT)
 
 /*
  * Indicates for which pages that are write-protected in the page table,
-- 
2.44.0