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Date: Wed, 27 Oct 2021 19:55:50 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Michal Hocko <mhocko@...e.com>,
Linux Memory Management List <linux-mm@...ck.org>,
Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Hellwig <hch@...radead.org>,
linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
Ilya Dryomov <idryomov@...il.com>,
Jeff Layton <jlayton@...nel.org>
Subject: Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL
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@...e.com>
> > > >
> > > > 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@...e.com>
> > > > ---
> > > > 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
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