Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL

[Uladzislau Rezki]

On Tue, Oct 26, 2021 at 09:33:15PM +0200, Uladzislau Rezki wrote:
> On Tue, Oct 26, 2021 at 06:28:52PM +0200, Michal Hocko wrote:
> > On Tue 26-10-21 17:48:32, Uladzislau Rezki wrote:
> > > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > >
> > > > Dave Chinner has mentioned that some of the xfs code would benefit from
> > > > kvmalloc support for __GFP_NOFAIL because they have allocations that
> > > > cannot fail and they do not fit into a single page.
> > > >
> > > > The larg part of the vmalloc implementation already complies with the
> > > > given gfp flags so there is no work for those to be done. The area
> > > > and page table allocations are an exception to that. Implement a retry
> > > > loop for those.
> > > >
> > > > Add a short sleep before retrying. 1 jiffy is a completely random
> > > > timeout. Ideally the retry would wait for an explicit event - e.g.
> > > > a change to the vmalloc space change if the failure was caused by
> > > > the space fragmentation or depletion. But there are multiple different
> > > > reasons to retry and this could become much more complex. Keep the retry
> > > > simple for now and just sleep to prevent from hogging CPUs.
> > > >
> > > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > > > ---
> > > >  mm/vmalloc.c | 10 +++++++++-
> > > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index c6cc77d2f366..602649919a9d 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -2941,8 +2941,12 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > > >         else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
> > > >                 flags = memalloc_noio_save();
> > > >
> > > > -       ret = vmap_pages_range(addr, addr + size, prot, area->pages,
> > > > +       do {
> > > > +               ret = vmap_pages_range(addr, addr + size, prot, area->pages,
> > > >                         page_shift);
> > > > +               if (ret < 0)
> > > > +                       schedule_timeout_uninterruptible(1);
> > > > +       } while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
> > > >
> > > 
> > > 1.
> > > After that change a below code:
> > > 
> > > <snip>
> > > if (ret < 0) {
> > >     warn_alloc(orig_gfp_mask, NULL,
> > >         "vmalloc error: size %lu, failed to map pages",
> > >         area->nr_pages * PAGE_SIZE);
> > >     goto fail;
> > > }
> > > <snip>
> > > 
> > > does not make any sense anymore.
> > 
> > Why? Allocations without __GFP_NOFAIL can still fail, no?
> > 
> Right. I meant one thing but wrote slightly differently. In case of
> vmap_pages_range() fails(if __GFP_NOFAIL is set) should we emit any
> warning message? Because either we can recover on a future iteration
> or it stuck there infinitely so a user does not understand what happened.
> From the other hand this is how __GFP_NOFAIL works, hm..
> 
> Another thing, i see that schedule_timeout_uninterruptible(1) is invoked
> for all cases even when __GFP_NOFAIL is not set, in that scenario we do
> not want to wait, instead we should return back to a caller asap. Or am
> i missing something here?
> 
> > > 2.
> > > Can we combine two places where we handle __GFP_NOFAIL into one place?
> > > That would look like as more sorted out.
> > 
> > I have to admit I am not really fluent at vmalloc code so I wanted to
> > make the code as simple as possible. How would I unwind all the allocated
> > memory (already allocated as GFP_NOFAIL) before retrying at
> > __vmalloc_node_range (if that is what you suggest). And isn't that a
> > bit wasteful?
> > 
> > Or did you have anything else in mind?
> >
> It depends on how often all this can fail. But let me double check if
> such combining is easy.
> 
I mean something like below. The idea is to not spread the __GFP_NOFAIL
across the vmalloc file keeping it in one solid place:

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d77830ff604c..f4b7927e217e 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2889,8 +2889,14 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 	unsigned long array_size;
 	unsigned int nr_small_pages = size >> PAGE_SHIFT;
 	unsigned int page_order;
+	unsigned long flags;
+	int ret;
 
 	array_size = (unsigned long)nr_small_pages * sizeof(struct page *);
+
+	/*
+	 * This is i do not understand why we do not want to see warning messages.
+	 */
 	gfp_mask |= __GFP_NOWARN;
 	if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
 		gfp_mask |= __GFP_HIGHMEM;
@@ -2930,8 +2936,23 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 		goto fail;
 	}
 
-	if (vmap_pages_range(addr, addr + size, prot, area->pages,
-			page_shift) < 0) {
+	/*
+	 * page tables allocations ignore external gfp mask, enforce it
+	 * by the scope API
+	 */
+	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
+		flags = memalloc_nofs_save();
+	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
+		flags = memalloc_noio_save();
+
+	ret = vmap_pages_range(addr, addr + size, prot, area->pages, page_shift);
+
+	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
+		memalloc_nofs_restore(flags);
+	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
+		memalloc_noio_restore(flags);
+
+	if (ret < 0) {
 		warn_alloc(gfp_mask, NULL,
 			"vmalloc error: size %lu, failed to map pages",
 			area->nr_pages * PAGE_SIZE);
@@ -2984,6 +3005,12 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
 		return NULL;
 	}
 
+	/*
+	 * Suppress all warnings for __GFP_NOFAIL allocation.
+	 */
+	if (gfp_mask & __GFP_NOFAIL)
+		gfp_mask |= __GFP_NOWARN;
+
 	if (vmap_allow_huge && !(vm_flags & VM_NO_HUGE_VMAP)) {
 		unsigned long size_per_node;
 
@@ -3010,16 +3037,22 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
 	area = __get_vm_area_node(real_size, align, shift, VM_ALLOC |
 				  VM_UNINITIALIZED | vm_flags, start, end, node,
 				  gfp_mask, caller);
-	if (!area) {
-		warn_alloc(gfp_mask, NULL,
-			"vmalloc error: size %lu, vm_struct allocation failed",
-			real_size);
-		goto fail;
-	}
+	if (area)
+		addr = __vmalloc_area_node(area, gfp_mask, prot, shift, node);
+
+	if (!area || !addr) {
+		if (gfp_mask & __GFP_NOFAIL) {
+			schedule_timeout_uninterruptible(1);
+			goto again;
+		}
+
+		if (!area)
+			warn_alloc(gfp_mask, NULL,
+				"vmalloc error: size %lu, vm_struct allocation failed",
+				real_size);
 
-	addr = __vmalloc_area_node(area, gfp_mask, prot, shift, node);
-	if (!addr)
 		goto fail;
+	}
 
 	/*
 	 * In this function, newly allocated vm_struct has VM_UNINITIALIZED
<snip>

--
Vlad Rezki