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Message-ID: <YBCexclveGV2KH1G@google.com>
Date: Tue, 26 Jan 2021 14:59:17 -0800
From: Minchan Kim <minchan@...nel.org>
To: Chris Goldsworthy <cgoldswo@...eaurora.org>
Cc: viro@...iv.linux.org.uk, Matthew Wilcox <willy@...radead.org>,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, Laura Abbott <lauraa@...eaurora.org>
Subject: Re: [PATCH v4] fs/buffer.c: Revoke LRU when trying to drop buffers
On Mon, Jan 25, 2021 at 10:58:30PM -0800, Chris Goldsworthy wrote:
> From: Laura Abbott <lauraa@...eaurora.org>
>
> 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@...eaurora.org>
> Signed-off-by: Chris Goldsworthy <cgoldswo@...eaurora.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Reported-by: kernel test robot <oliver.sang@...el.com>
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)
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>
>
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