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Message-ID: <871qk582tn.fsf@toke.dk>
Date: Thu, 27 Apr 2023 22:53:08 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Jesper Dangaard Brouer <brouer@...hat.com>,
Ilias Apalodimas <ilias.apalodimas@...aro.org>,
netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
linux-mm@...ck.org, Mel Gorman <mgorman@...hsingularity.net>
Cc: Jesper Dangaard Brouer <brouer@...hat.com>, lorenzo@...nel.org,
linyunsheng@...wei.com, bpf@...r.kernel.org,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org
Subject: Re: [PATCH RFC net-next/mm V2 1/2] page_pool: Remove workqueue in
new shutdown scheme
Jesper Dangaard Brouer <brouer@...hat.com> writes:
> This removes the workqueue scheme that periodically tests when
> inflight reach zero such that page_pool memory can be freed.
>
> This change adds code to fast-path free checking for a shutdown flags
> bit after returning PP pages.
I think the general approach is workable, but spotted a few issues with
the details, see below.
> Performance is very important for PP, as the fast path is used for
> XDP_DROP use-cases where NIC drivers recycle PP pages directly into PP
> alloc cache.
>
> The goal were that this code change should have zero impact on this
> fast-path. The slight code reorg of likely() are deliberate. Micro
> benchmarking done via kernel module[1] on x86_64, shows this code
> change only cost a single instruction extra (approx 0.3 nanosec on CPU
> E5-1650 @3.60GHz).
>
> It is possible to make this code zero impact via static_key, but that
> change is not considered worth the complexity.
>
> [1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
> include/net/page_pool.h | 9 ++--
> net/core/page_pool.c | 100 +++++++++++++++++++++++++++++------------------
> 2 files changed, 66 insertions(+), 43 deletions(-)
>
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index c8ec2f34722b..a71c0f2695b0 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -50,6 +50,9 @@
> PP_FLAG_DMA_SYNC_DEV |\
> PP_FLAG_PAGE_FRAG)
>
> +/* Internal flag: PP in shutdown phase, waiting for inflight pages */
> +#define PP_FLAG_SHUTDOWN BIT(8)
> +
> /*
> * Fast allocation side cache array/stack
> *
> @@ -151,11 +154,6 @@ static inline u64 *page_pool_ethtool_stats_get(u64 *data, void *stats)
> struct page_pool {
> struct page_pool_params p;
>
> - struct delayed_work release_dw;
> - void (*disconnect)(void *);
> - unsigned long defer_start;
> - unsigned long defer_warn;
> -
> u32 pages_state_hold_cnt;
> unsigned int frag_offset;
> struct page *frag_page;
> @@ -165,6 +163,7 @@ struct page_pool {
> /* these stats are incremented while in softirq context */
> struct page_pool_alloc_stats alloc_stats;
> #endif
> + void (*disconnect)(void *);
> u32 xdp_mem_id;
>
> /*
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index e212e9d7edcb..b8359d84e30f 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -23,9 +23,6 @@
>
> #include <trace/events/page_pool.h>
>
> -#define DEFER_TIME (msecs_to_jiffies(1000))
> -#define DEFER_WARN_INTERVAL (60 * HZ)
> -
> #define BIAS_MAX LONG_MAX
>
> #ifdef CONFIG_PAGE_POOL_STATS
> @@ -380,6 +377,10 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
> struct page *page;
> int i, nr_pages;
>
> + /* API usage BUG: PP in shutdown phase, cannot alloc new pages */
> + if (WARN_ON(pool->p.flags & PP_FLAG_SHUTDOWN))
> + return NULL;
> +
> /* Don't support bulk alloc for high-order pages */
> if (unlikely(pp_order))
> return __page_pool_alloc_page_order(pool, gfp);
> @@ -445,15 +446,20 @@ struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp)
> }
> EXPORT_SYMBOL(page_pool_alloc_pages);
>
> +/* Avoid inlining code to avoid speculative fetching cacheline */
> +noinline u32 pp_read_hold_cnt(struct page_pool *pool)
> +{
> + return READ_ONCE(pool->pages_state_hold_cnt);
> +}
> +
> /* Calculate distance between two u32 values, valid if distance is below 2^(31)
> * https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
> */
> #define _distance(a, b) (s32)((a) - (b))
>
> -static s32 page_pool_inflight(struct page_pool *pool)
> +static s32 __page_pool_inflight(struct page_pool *pool,
> + u32 hold_cnt, u32 release_cnt)
> {
> - u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
> - u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt);
> s32 inflight;
>
> inflight = _distance(hold_cnt, release_cnt);
> @@ -464,6 +470,16 @@ static s32 page_pool_inflight(struct page_pool *pool)
> return inflight;
> }
>
> +static s32 page_pool_inflight(struct page_pool *pool)
> +{
> + u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt);
> + u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
> + return __page_pool_inflight(pool, hold_cnt, release_cnt);
> +}
> +
> +static int page_pool_free_attempt(struct page_pool *pool,
> + u32 hold_cnt, u32 release_cnt);
> +
> /* Disconnects a page (from a page_pool). API users can have a need
> * to disconnect a page (from a page_pool), to allow it to be used as
> * a regular page (that will eventually be returned to the normal
> @@ -471,8 +487,10 @@ static s32 page_pool_inflight(struct page_pool *pool)
> */
> void page_pool_release_page(struct page_pool *pool, struct page *page)
> {
> + unsigned int flags = READ_ONCE(pool->p.flags);
> dma_addr_t dma;
> - int count;
> + u32 release_cnt;
> + u32 hold_cnt;
>
> if (!(pool->p.flags & PP_FLAG_DMA_MAP))
> /* Always account for inflight pages, even if we didn't
> @@ -490,11 +508,15 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
> skip_dma_unmap:
> page_pool_clear_pp_info(page);
>
> - /* This may be the last page returned, releasing the pool, so
> - * it is not safe to reference pool afterwards.
> - */
> - count = atomic_inc_return_relaxed(&pool->pages_state_release_cnt);
> - trace_page_pool_state_release(pool, page, count);
> + if (flags & PP_FLAG_SHUTDOWN)
> + hold_cnt = pp_read_hold_cnt(pool);
> +
> + release_cnt = atomic_inc_return(&pool->pages_state_release_cnt);
> + trace_page_pool_state_release(pool, page, release_cnt);
> +
> + /* In shutdown phase, last page will free pool instance */
> + if (flags & PP_FLAG_SHUTDOWN)
> + page_pool_free_attempt(pool, hold_cnt, release_cnt);
Since the assumption is that no new pages will be allocated once the
PP_FLAG_SHUTDOWN is set (i.e., hold_count can not increase in the case),
I don't think it matters what order you read the hold and release counts
in? So you could simplify the above to just:
> + if (flags & PP_FLAG_SHUTDOWN)
> + page_pool_free_attempt(pool, pp_read_hold_cnt(pool), release_cnt);
and drop the second check of the flag further up?
You could probably even lose the hold_cnt argument entirely from
page_pool_free_attempt() and just have it call pp_read_hold_cnt() directly?
> }
> EXPORT_SYMBOL(page_pool_release_page);
>
> @@ -535,7 +557,7 @@ static bool page_pool_recycle_in_ring(struct page_pool *pool, struct page *page)
> static bool page_pool_recycle_in_cache(struct page *page,
> struct page_pool *pool)
> {
> - if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE)) {
> + if (pool->alloc.count == PP_ALLOC_CACHE_SIZE) {
> recycle_stat_inc(pool, cache_full);
> return false;
> }
> @@ -546,6 +568,8 @@ static bool page_pool_recycle_in_cache(struct page *page,
> return true;
> }
>
> +static void page_pool_empty_ring(struct page_pool *pool);
> +
> /* If the page refcnt == 1, this will try to recycle the page.
> * if PP_FLAG_DMA_SYNC_DEV is set, we'll try to sync the DMA area for
> * the configured size min(dma_sync_size, pool->max_len).
> @@ -572,7 +596,8 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
> page_pool_dma_sync_for_device(pool, page,
> dma_sync_size);
>
> - if (allow_direct && in_softirq() &&
> + /* During PP shutdown, no direct recycle must occur */
> + if (likely(allow_direct && in_softirq()) &&
> page_pool_recycle_in_cache(page, pool))
> return NULL;
>
> @@ -609,6 +634,8 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
> recycle_stat_inc(pool, ring_full);
> page_pool_return_page(pool, page);
> }
> + if (pool->p.flags & PP_FLAG_SHUTDOWN)
> + page_pool_empty_ring(pool);
> }
> EXPORT_SYMBOL(page_pool_put_defragged_page);
>
> @@ -648,13 +675,17 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data,
>
> /* Hopefully all pages was return into ptr_ring */
> if (likely(i == bulk_len))
> - return;
> + goto out;
>
> /* ptr_ring cache full, free remaining pages outside producer lock
> * since put_page() with refcnt == 1 can be an expensive operation
> */
> for (; i < bulk_len; i++)
> page_pool_return_page(pool, data[i]);
> +
> +out:
> + if (pool->p.flags & PP_FLAG_SHUTDOWN)
> + page_pool_empty_ring(pool);
> }
> EXPORT_SYMBOL(page_pool_put_page_bulk);
>
> @@ -737,6 +768,7 @@ struct page *page_pool_alloc_frag(struct page_pool *pool,
> }
> EXPORT_SYMBOL(page_pool_alloc_frag);
>
> +noinline
> static void page_pool_empty_ring(struct page_pool *pool)
> {
> struct page *page;
> @@ -796,39 +828,29 @@ static void page_pool_scrub(struct page_pool *pool)
> page_pool_empty_ring(pool);
> }
So this is not in the diff context, but page_pool_empty_ring() does
this:
static void page_pool_empty_ring(struct page_pool *pool)
{
struct page *page;
/* Empty recycle ring */
while ((page = ptr_ring_consume_bh(&pool->ring))) {
/* Verify the refcnt invariant of cached pages */
if (!(page_ref_count(page) == 1))
pr_crit("%s() page_pool refcnt %d violation\n",
__func__, page_ref_count(page));
page_pool_return_page(pool, page);
}
}
...and with this patch, that page_pool_return_page() call will now free
the pool memory entirely when the last page is returned. When it does
this, the condition in the while loop will still execute afterwards; it
would return false, but if the pool was freed, it's now referencing
freed memory when trying to read from pool->ring.
So I think page_pool_empty_ring needs to either pull out all the pages
in the ring to an on-stack buffer before calling page_pool_return_page()
on them, or there needs to be some other way to break the loop early.
There are a couple of other places where page_pool_return_page() is
called in a loop where the loop variable lives inside struct page_pool,
so we need to be absolutely sure they will never be called in the
shutdown stage, or they'll have to be fixed as well.
>
> -static int page_pool_release(struct page_pool *pool)
> +noinline
> +static int page_pool_free_attempt(struct page_pool *pool,
> + u32 hold_cnt, u32 release_cnt)
> {
> int inflight;
>
> - page_pool_scrub(pool);
> - inflight = page_pool_inflight(pool);
> + inflight = __page_pool_inflight(pool, hold_cnt, release_cnt);
> if (!inflight)
> page_pool_free(pool);
>
> return inflight;
> }
>
> -static void page_pool_release_retry(struct work_struct *wq)
> +static int page_pool_release(struct page_pool *pool)
> {
> - struct delayed_work *dwq = to_delayed_work(wq);
> - struct page_pool *pool = container_of(dwq, typeof(*pool), release_dw);
> int inflight;
>
> - inflight = page_pool_release(pool);
> + page_pool_scrub(pool);
> + inflight = page_pool_inflight(pool);
> if (!inflight)
> - return;
> -
> - /* Periodic warning */
> - if (time_after_eq(jiffies, pool->defer_warn)) {
> - int sec = (s32)((u32)jiffies - (u32)pool->defer_start) / HZ;
> -
> - pr_warn("%s() stalled pool shutdown %d inflight %d sec\n",
> - __func__, inflight, sec);
> - pool->defer_warn = jiffies + DEFER_WARN_INTERVAL;
> - }
> + page_pool_free(pool);
>
> - /* Still not ready to be disconnected, retry later */
> - schedule_delayed_work(&pool->release_dw, DEFER_TIME);
> + return inflight;
> }
>
> void page_pool_use_xdp_mem(struct page_pool *pool, void (*disconnect)(void *),
> @@ -868,11 +890,13 @@ void page_pool_destroy(struct page_pool *pool)
> if (!page_pool_release(pool))
> return;
>
> - pool->defer_start = jiffies;
> - pool->defer_warn = jiffies + DEFER_WARN_INTERVAL;
> + /* PP have pages inflight, thus cannot immediately release memory.
> + * Enter into shutdown phase.
> + */
> + pool->p.flags |= PP_FLAG_SHUTDOWN;
I think there's another race here: once the flag is set in this line
(does this need a memory barrier, BTW?), another CPU can return the last
outstanding page, read the flag and call page_pool_empty_ring(). If this
happens before the call to page_pool_empty_ring() below, you'll get a
use-after-free.
To avoid this, we could artificially bump the pool->hold_cnt *before*
setting the flag above; that way we know that the page_pool_empty_ring()
won't trigger a release, because inflight pages will never go below 1.
And then, below the page_pool_empty_ring() call below, we can add an
artificial bump of the release_cnt as well, which means we'll get proper
atomic semantics on the counters and only ever release once. I.e.,:
> - INIT_DELAYED_WORK(&pool->release_dw, page_pool_release_retry);
> - schedule_delayed_work(&pool->release_dw, DEFER_TIME);
> + /* Concurrent CPUs could have returned last pages into ptr_ring */
> + page_pool_empty_ring(pool);
release_cnt = atomic_inc_return(&pool->pages_state_release_cnt);
page_pool_free_attempt(pool, release_cnt);
-Toke
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