The ethernet driver may allocate skb (and skb->data) via napi_alloc_skb().
This ends up to page_frag_alloc() to allocate skb->data from
page_frag_cache->va.
During the memory pressure, page_frag_cache->va may be allocated as
pfmemalloc page. As a result, the skb->pfmemalloc is always true as
skb->data is from page_frag_cache->va. The skb will be dropped if the
sock (receiver) does not have SOCK_MEMALLOC. This is expected behaviour
under memory pressure.
However, once kernel is not under memory pressure any longer (suppose large
amount of memory pages are just reclaimed), the page_frag_alloc() may still
re-use the prior pfmemalloc page_frag_cache->va to allocate skb->data. As a
result, the skb->pfmemalloc is always true unless page_frag_cache->va is
re-allocated, even if the kernel is not under memory pressure any longer.
Here is how kernel runs into issue.
1. The kernel is under memory pressure and allocation of
PAGE_FRAG_CACHE_MAX_ORDER in __page_frag_cache_refill() will fail. Instead,
the pfmemalloc page is allocated for page_frag_cache->va.
2: All skb->data from page_frag_cache->va (pfmemalloc) will have
skb->pfmemalloc=true. The skb will always be dropped by sock without
SOCK_MEMALLOC. This is an expected behaviour.
3. Suppose a large amount of pages are reclaimed and kernel is not under
memory pressure any longer. We expect skb->pfmemalloc drop will not happen.
4. Unfortunately, page_frag_alloc() does not proactively re-allocate
page_frag_alloc->va and will always re-use the prior pfmemalloc page. The
skb->pfmemalloc is always true even kernel is not under memory pressure any
longer.
Fix this by freeing and re-allocating the page instead of recycling it.
References: https://lore.kernel.org/lkml/[email protected]/
References: https://lore.kernel.org/linux-mm/[email protected]/
Suggested-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Aruna Ramakrishna <[email protected]>
Cc: Bert Barbe <[email protected]>
Cc: Rama Nichanamatlu <[email protected]>
Cc: Venkat Venkatsubra <[email protected]>
Cc: Manjunath Patil <[email protected]>
Cc: Joe Jin <[email protected]>
Cc: SRINIVAS <[email protected]>
Cc: [email protected]
Fixes: 79930f5892e ("net: do not deplete pfmemalloc reserve")
Signed-off-by: Dongli Zhang <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
Changed since v1:
- change author from Matthew to Dongli
- Add references to all prior discussions
- Add more details to commit message
Changed since v2:
- add unlikely (suggested by Eric Dumazet)
mm/page_alloc.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 23f5066bd4a5..91129ce75ed4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5103,6 +5103,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
goto refill;
+ if (unlikely(nc->pfmemalloc)) {
+ free_the_page(page, compound_order(page));
+ goto refill;
+ }
+
#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
/* if size can vary use size else just use PAGE_SIZE */
size = nc->size;
--
2.17.1
On Sun, Nov 15, 2020 at 9:16 PM Dongli Zhang <[email protected]> wrote:
>
> The ethernet driver may allocate skb (and skb->data) via napi_alloc_skb().
> This ends up to page_frag_alloc() to allocate skb->data from
> page_frag_cache->va.
>
> During the memory pressure, page_frag_cache->va may be allocated as
> pfmemalloc page. As a result, the skb->pfmemalloc is always true as
> skb->data is from page_frag_cache->va. The skb will be dropped if the
> sock (receiver) does not have SOCK_MEMALLOC. This is expected behaviour
> under memory pressure.
...
> References: https://lore.kernel.org/lkml/[email protected]/
> References: https://lore.kernel.org/linux-mm/[email protected]/
> Suggested-by: Matthew Wilcox (Oracle) <[email protected]>
> Cc: Aruna Ramakrishna <[email protected]>
> Cc: Bert Barbe <[email protected]>
> Cc: Rama Nichanamatlu <[email protected]>
> Cc: Venkat Venkatsubra <[email protected]>
> Cc: Manjunath Patil <[email protected]>
> Cc: Joe Jin <[email protected]>
> Cc: SRINIVAS <[email protected]>
> Cc: [email protected]
> Fixes: 79930f5892e ("net: do not deplete pfmemalloc reserve")
> Signed-off-by: Dongli Zhang <[email protected]>
> Acked-by: Vlastimil Babka <[email protected]>
> ---
> Changed since v1:
> - change author from Matthew to Dongli
> - Add references to all prior discussions
> - Add more details to commit message
> Changed since v2:
> - add unlikely (suggested by Eric Dumazet)
>
> mm/page_alloc.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 23f5066bd4a5..91129ce75ed4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5103,6 +5103,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
> if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
> goto refill;
>
> + if (unlikely(nc->pfmemalloc)) {
> + free_the_page(page, compound_order(page));
> + goto refill;
> + }
> +
Reviewed-by: Eric Dumazet <[email protected]>
Thanks !
On Sun, 15 Nov 2020 12:10:29 -0800 Dongli Zhang wrote:
> The ethernet driver may allocate skb (and skb->data) via napi_alloc_skb().
> This ends up to page_frag_alloc() to allocate skb->data from
> page_frag_cache->va.
>
> During the memory pressure, page_frag_cache->va may be allocated as
> pfmemalloc page. As a result, the skb->pfmemalloc is always true as
> skb->data is from page_frag_cache->va. The skb will be dropped if the
> sock (receiver) does not have SOCK_MEMALLOC. This is expected behaviour
> under memory pressure.
>
> However, once kernel is not under memory pressure any longer (suppose large
> amount of memory pages are just reclaimed), the page_frag_alloc() may still
> re-use the prior pfmemalloc page_frag_cache->va to allocate skb->data. As a
> result, the skb->pfmemalloc is always true unless page_frag_cache->va is
> re-allocated, even if the kernel is not under memory pressure any longer.
>
> Here is how kernel runs into issue.
>
> 1. The kernel is under memory pressure and allocation of
> PAGE_FRAG_CACHE_MAX_ORDER in __page_frag_cache_refill() will fail. Instead,
> the pfmemalloc page is allocated for page_frag_cache->va.
>
> 2: All skb->data from page_frag_cache->va (pfmemalloc) will have
> skb->pfmemalloc=true. The skb will always be dropped by sock without
> SOCK_MEMALLOC. This is an expected behaviour.
>
> 3. Suppose a large amount of pages are reclaimed and kernel is not under
> memory pressure any longer. We expect skb->pfmemalloc drop will not happen.
>
> 4. Unfortunately, page_frag_alloc() does not proactively re-allocate
> page_frag_alloc->va and will always re-use the prior pfmemalloc page. The
> skb->pfmemalloc is always true even kernel is not under memory pressure any
> longer.
>
> Fix this by freeing and re-allocating the page instead of recycling it.
Andrew, are you taking this via -mm or should I put it in net?
I'm sending a PR to Linus tomorrow.
On Wed, 18 Nov 2020 11:46:54 -0800 Jakub Kicinski <[email protected]> wrote:
> > 1. The kernel is under memory pressure and allocation of
> > PAGE_FRAG_CACHE_MAX_ORDER in __page_frag_cache_refill() will fail. Instead,
> > the pfmemalloc page is allocated for page_frag_cache->va.
> >
> > 2: All skb->data from page_frag_cache->va (pfmemalloc) will have
> > skb->pfmemalloc=true. The skb will always be dropped by sock without
> > SOCK_MEMALLOC. This is an expected behaviour.
> >
> > 3. Suppose a large amount of pages are reclaimed and kernel is not under
> > memory pressure any longer. We expect skb->pfmemalloc drop will not happen.
> >
> > 4. Unfortunately, page_frag_alloc() does not proactively re-allocate
> > page_frag_alloc->va and will always re-use the prior pfmemalloc page. The
> > skb->pfmemalloc is always true even kernel is not under memory pressure any
> > longer.
> >
> > Fix this by freeing and re-allocating the page instead of recycling it.
>
> Andrew, are you taking this via -mm or should I put it in net?
> I'm sending a PR to Linus tomorrow.
Please go ahead - if/when it appears in mainline or linux-next, I'll
drop the -mm copy.
On Wed, 18 Nov 2020 13:13:35 -0800 Andrew Morton wrote:
> On Wed, 18 Nov 2020 11:46:54 -0800 Jakub Kicinski <[email protected]> wrote:
>
> > > 1. The kernel is under memory pressure and allocation of
> > > PAGE_FRAG_CACHE_MAX_ORDER in __page_frag_cache_refill() will fail. Instead,
> > > the pfmemalloc page is allocated for page_frag_cache->va.
> > >
> > > 2: All skb->data from page_frag_cache->va (pfmemalloc) will have
> > > skb->pfmemalloc=true. The skb will always be dropped by sock without
> > > SOCK_MEMALLOC. This is an expected behaviour.
> > >
> > > 3. Suppose a large amount of pages are reclaimed and kernel is not under
> > > memory pressure any longer. We expect skb->pfmemalloc drop will not happen.
> > >
> > > 4. Unfortunately, page_frag_alloc() does not proactively re-allocate
> > > page_frag_alloc->va and will always re-use the prior pfmemalloc page. The
> > > skb->pfmemalloc is always true even kernel is not under memory pressure any
> > > longer.
> > >
> > > Fix this by freeing and re-allocating the page instead of recycling it.
> >
> > Andrew, are you taking this via -mm or should I put it in net?
> > I'm sending a PR to Linus tomorrow.
>
> Please go ahead - if/when it appears in mainline or linux-next, I'll
> drop the -mm copy.
Okay, applied, thank you!