linux-kernel - Re: [PATCH 6/6] mm: Add memalloc

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Message-ID: <20200625124017.GL1320@dhcp22.suse.cz>
Date:   Thu, 25 Jun 2020 14:40:17 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, dm-devel@...hat.com,
        Mikulas Patocka <mpatocka@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.de>
Subject: Re: [PATCH 6/6] mm: Add memalloc_nowait

On Thu 25-06-20 12:31:22, Matthew Wilcox 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.

memalloc_nowait is a good idea! I suspect the primary usecase would be
vmalloc.

> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

[...]
> @@ -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);

This looks confusing though. I am not familiar with alloc_buffer and
there is quite some tweaking around __GFP_NORETRY in alloc_buffer_data
which I do not follow but GFP_KERNEL just struck my eyes. So why cannot
we have 
		alloc_buffer(GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);
-- 
Michal Hocko
SUSE Labs