Re: [PATCH v4] fs/buffer.c: Revoke LRU when trying to drop buffers

[Minchan Kim]

On Mon, Jan 25, 2021 at 10:58:30PM -0800, Chris Goldsworthy wrote:
> From: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
> 
> When a buffer is added to the LRU list, a reference is taken which is
> not dropped until the buffer is evicted from the LRU list. This is the
> correct behavior, however this LRU reference will prevent the buffer
> from being dropped. This means that the buffer can't actually be dropped
> until it is selected for eviction. There's no bound on the time spent
> on the LRU list, which means that the buffer may be undroppable for
> very long periods of time. Given that migration involves dropping
> buffers, the associated page is now unmigratible for long periods of
> time as well. CMA relies on being able to migrate a specific range
> of pages, so these types of failures make CMA significantly
> less reliable, especially under high filesystem usage.
> 
> Rather than waiting for the LRU algorithm to eventually kick out
> the buffer, explicitly remove the buffer from the LRU list when trying
> to drop it. There is still the possibility that the buffer
> could be added back on the list, but that indicates the buffer is
> still in use and would probably have other 'in use' indicates to
> prevent dropping.
> 
> Note: a bug reported by "kernel test robot" lead to a switch from
> using xas_for_each() to xa_for_each().
> 
> Signed-off-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
> Signed-off-by: Chris Goldsworthy <cgoldswo@xxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>

Hi Chris,

The release buffer_head in LRU is great improvement for migration
point of view.

A question: 

Can't we invalidate(e.g., invalidate_bh_lrus) bh_lru in migrate_prep or
elsewhere when migration found the failure and is about to retry?

Migration has done such a way for other per-cpu stuffs for a long time,
which would be more consistent with others and might be faster sometimes
with reducing IPI calls for page.


> ---
>  fs/buffer.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 74 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 96c7604..27516a0 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -48,6 +48,7 @@
>  #include <linux/sched/mm.h>
>  #include <trace/events/block.h>
>  #include <linux/fscrypt.h>
> +#include <linux/xarray.h>
>  
>  #include "internal.h"
>  
> @@ -1471,12 +1472,55 @@ static bool has_bh_in_lru(int cpu, void *dummy)
>  	return false;
>  }
>  
> +static void __evict_bhs_lru(void *arg)
> +{
> +	struct bh_lru *b = &get_cpu_var(bh_lrus);
> +	struct xarray *busy_bhs = arg;
> +	struct buffer_head *bh;
> +	unsigned long i, xarray_index;
> +
> +	xa_for_each(busy_bhs, xarray_index, bh) {
> +		for (i = 0; i < BH_LRU_SIZE; i++) {
> +			if (b->bhs[i] == bh) {
> +				brelse(b->bhs[i]);
> +				b->bhs[i] = NULL;
> +				break;
> +			}
> +		}
> +	}
> +
> +	put_cpu_var(bh_lrus);
> +}
> +
> +static bool page_has_bhs_in_lru(int cpu, void *arg)
> +{
> +	struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
> +	struct xarray *busy_bhs = arg;
> +	struct buffer_head *bh;
> +	unsigned long i, xarray_index;
> +
> +	xa_for_each(busy_bhs, xarray_index, bh) {
> +		for (i = 0; i < BH_LRU_SIZE; i++) {
> +			if (b->bhs[i] == bh)
> +				return true;
> +		}
> +	}
> +
> +	return false;
> +
> +}
>  void invalidate_bh_lrus(void)
>  {
>  	on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);
>  }
>  EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
>  
> +static void evict_bh_lrus(struct xarray *busy_bhs)
> +{
> +	on_each_cpu_cond(page_has_bhs_in_lru, __evict_bhs_lru,
> +			 busy_bhs, 1);
> +}
> +
>  void set_bh_page(struct buffer_head *bh,
>  		struct page *page, unsigned long offset)
>  {
> @@ -3242,14 +3286,38 @@ drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
>  {
>  	struct buffer_head *head = page_buffers(page);
>  	struct buffer_head *bh;
> +	struct xarray busy_bhs;
> +	int bh_count = 0;
> +	int xa_ret, ret = 0;
> +
> +	xa_init(&busy_bhs);
>  
>  	bh = head;
>  	do {
> -		if (buffer_busy(bh))
> -			goto failed;
> +		if (buffer_busy(bh)) {
> +			xa_ret = xa_err(xa_store(&busy_bhs, bh_count++,
> +						 bh, GFP_ATOMIC));
> +			if (xa_ret)
> +				goto out;
> +		}
>  		bh = bh->b_this_page;
>  	} while (bh != head);
>  
> +	if (bh_count) {
> +		/*
> +		 * Check if the busy failure was due to an outstanding
> +		 * LRU reference
> +		 */
> +		evict_bh_lrus(&busy_bhs);
> +		do {
> +			if (buffer_busy(bh))
> +				goto out;
> +
> +			bh = bh->b_this_page;
> +		} while (bh != head);
> +	}
> +
> +	ret = 1;
>  	do {
>  		struct buffer_head *next = bh->b_this_page;
>  
> @@ -3259,9 +3327,10 @@ drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
>  	} while (bh != head);
>  	*buffers_to_free = head;
>  	detach_page_private(page);
> -	return 1;
> -failed:
> -	return 0;
> +out:
> +	xa_destroy(&busy_bhs);
> +
> +	return ret;
>  }
>  
>  int try_to_free_buffers(struct page *page)
> -- 
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> a Linux Foundation Collaborative Project
> 
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