Re: [PATCH 6/6] mm: Add memalloc_nowait

Daniel Vetter
2020-10-23T14:49:55Z

On Thu, Jun 25, 2020 at 12:31:22PM +0100, Matthew Wilcox (Oracle) wrote:
> Similar to memalloc_noio() and memalloc_nofs(), memalloc_nowait()
> guarantees we will not sleep to reclaim memory. Use it to simplify
> dm-bufio's allocations.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> drivers/md/dm-bufio.c | 30 ++++++++----------------------
> include/linux/sched.h | 1 +
> include/linux/sched/mm.h | 12 ++++++++----
> 3 files changed, 17 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> index 6d1565021d74..140ada9a2c8f 100644
> --- a/drivers/md/dm-bufio.c
> +++ b/drivers/md/dm-bufio.c
> @@ -412,23 +412,6 @@ static void *alloc_buffer_data(struct dm_bufio_client *c, gfp_t gfp_mask,
>
> *data_mode = DATA_MODE_VMALLOC;
>
> - /*
> - * __vmalloc allocates the data pages and auxiliary structures with
> - * gfp_flags that were specified, but pagetables are always allocated
> - * with GFP_KERNEL, no matter what was specified as gfp_mask.
> - *
> - * Consequently, we must set per-process flag PF_MEMALLOC_NOIO so that
> - * all allocations done by this process (including pagetables) are done
> - * as if GFP_NOIO was specified.
> - */
> - if (gfp_mask & __GFP_NORETRY) {
> - unsigned noio_flag = memalloc_noio_save();
> - void *ptr = __vmalloc(c->block_size, gfp_mask);
> -
> - memalloc_noio_restore(noio_flag);
> - return ptr;
> - }
> -
> return __vmalloc(c->block_size, gfp_mask);
> }
>
> @@ -866,9 +849,6 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
> * dm-bufio is resistant to allocation failures (it just keeps
> * one buffer reserved in cases all the allocations fail).
> * So set flags to not try too hard:
> - * GFP_NOWAIT: don't wait; if we need to sleep we'll release our
> - * mutex and wait ourselves.
> - * __GFP_NORETRY: don't retry and rather return failure
> * __GFP_NOMEMALLOC: don't use emergency reserves
> * __GFP_NOWARN: don't print a warning in case of failure
> *
> @@ -877,7 +857,9 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
> */
> while (1) {
> if (dm_bufio_cache_size_latch != 1) {
> - b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> + unsigned nowait_flag = memalloc_nowait_save();
> + b = alloc_buffer(c, GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN);
> + memalloc_nowait_restore(nowait_flag);
> if (b)
> return b;
> }
> @@ -886,8 +868,12 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
> return NULL;
>
> if (dm_bufio_cache_size_latch != 1 && !tried_noio_alloc) {
> + unsigned noio_flag;
> +
> dm_bufio_unlock(c);
> - b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> + noio_flag = memalloc_noio_save();
> + b = alloc_buffer(c, GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN);
> + memalloc_noio_restore(noio_flag);
> dm_bufio_lock(c);
> if (b)
> return b;
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 90336850e940..b1c2cddd366c 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -803,6 +803,7 @@ struct task_struct {
> #endif
> unsigned memalloc_noio:1;
> unsigned memalloc_nofs:1;
> + unsigned memalloc_nowait:1;

I think you also need to update gfpflags_allow_blocking() to take your new
flag into account, or some debug checks all over the place might misfire.

Plus I have a patch which rolls this out to a few more places in all the
allocators.
-Daniel

> unsigned memalloc_nocma:1;
>
> unsigned long atomic_flags; /* Flags requiring atomic access. */
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 6f7b59a848a6..6484569f50df 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -179,12 +179,16 @@ static inline bool in_vfork(struct task_struct *tsk)
> static inline gfp_t current_gfp_context(gfp_t flags)
> {
> if (unlikely(current->memalloc_noio || current->memalloc_nofs ||
> - current->memalloc_nocma)) {
> + current->memalloc_nocma) || current->memalloc_nowait) {
> /*
> - * NOIO implies both NOIO and NOFS and it is a weaker context
> - * so always make sure it makes precedence
> + * Clearing DIRECT_RECLAIM means we won't get to the point
> + * of testing IO or FS, so we don't need to bother clearing
> + * them. noio implies neither IO nor FS and it is a weaker
> + * context so always make sure it takes precedence.
> */
> - if (current->memalloc_noio)
> + if (current->memalloc_nowait)
> + flags &= ~__GFP_DIRECT_RECLAIM;
> + else if (current->memalloc_noio)
> flags &= ~(__GFP_IO | __GFP_FS);
> else if (current->memalloc_nofs)
> flags &= ~__GFP_FS;
> --
> 2.27.0
>
>

--
Daniel Vetter
Software Engineer, Intel Corporation