linux-kernel - Re: [PATCH v2] mm: prototype: rid swapoff of quadratic complexity

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Date:   Mon, 26 Nov 2018 13:35:30 -0500
From:   Vineeth Remanan Pillai <vpillai@...italocean.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kelley Nielsen <kelleynnn@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2] mm: prototype: rid swapoff of quadratic complexity

Hi Mathew,


Thanks for your response!

On 11/26/18 12:22 PM, Matthew Wilcox wrote:
> On Mon, Nov 26, 2018 at 04:55:21PM +0000, Vineeth Remanan Pillai wrote:
>> +	do {
>> +		XA_STATE(xas, &mapping->i_pages, start);
>> +		int i;
>> +		int entries = 0;
>> +		struct page *page;
>> +		pgoff_t indices[PAGEVEC_SIZE];
>> +		unsigned long end = start + PAGEVEC_SIZE;
>>   
>> +		rcu_read_lock();
>> +		xas_for_each(&xas, page, end) {
> I think this is a mistake.  You should probably specify ULONG_MAX for the
> end.  Otherwise if there are no swap entries in the first 60kB of the file,
> you'll just exit.  That does mean you'll need to check 'entries' for
> hitting PAGEVEC_SIZE.

Thanks for pointing this out. I shall fix this in the next version.

> This seems terribly complicated.  You run through i_pages, record the
> indices of the swap entries, then go back and look them up again by
> calling shmem_getpage() which calls the incredibly complex 300 line
> shmem_getpage_gfp().
>
> Can we refactor shmem_getpage_gfp() to skip some of the checks which
> aren't necessary when called from this path, and turn this into a nice
> simple xas_for_each() loop which works one entry at a time?

I shall investigate this and make this simpler as you suggested.

>> +	list_for_each_safe(p, next, &shmem_swaplist) {
>> +		info = list_entry(p, struct shmem_inode_info, swaplist);
> This could use list_for_each_entry_safe(), right?

Yes, you are right. Will fix.


Thanks,

Vineeth

>