Linux-mm Archive on lore.kernel.org
 help / color / Atom feed
From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE
Date: Thu, 1 Nov 2018 19:42:48 +0300
Message-ID: <ae51e16b-459c-7d59-6277-b1a197dbf5ff@yandex-team.ru> (raw)
In-Reply-To: <20181101125543.GH23921@dhcp22.suse.cz>

On 01.11.2018 15:55, Michal Hocko wrote:
> On Thu 01-11-18 13:48:17, Konstantin Khlebnikov wrote:
>>
>>
>> On 01.11.2018 13:24, Michal Hocko wrote:
>>> On Thu 01-11-18 13:09:16, Konstantin Khlebnikov wrote:
>>>> Allocations over KMALLOC_MAX_SIZE could be served only by vmalloc.
>>>
>>> I would go on and say that allocations with sizes too large can actually
>>> trigger a warning (once you have posted in the previous version outside
>>> of the changelog area) because that might be interesting to people -
>>> there are deployments to panic on warning and then a warning is much
>>> more important.
>>
>> It seems that warning isn't completely valid.
>>
>>
>> __alloc_pages_slowpath() handles this more gracefully:
>>
>> 	/*
>> 	 * In the slowpath, we sanity check order to avoid ever trying to
>> 	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
>> 	 * be using allocators in order of preference for an area that is
>> 	 * too large.
>> 	 */
>> 	if (order >= MAX_ORDER) {
>> 		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
>> 		return NULL;
>> 	}
>>
>>
>> Fast path is ready for order >= MAX_ORDER
>>
>>
>> Problem is in node_reclaim() which is called earlier than __alloc_pages_slowpath()
>> from surprising place - get_page_from_freelist()
>>
>>
>> Probably node_reclaim() simply needs something like this:
>>
>> 	if (order >= MAX_ORDER)
>> 		return NODE_RECLAIM_NOSCAN;
> 
> Maybe but the point is that triggering this warning is possible. Even if
> the warning is bogus it doesn't really make much sense to even try
> kmalloc if the size is not supported by the allocator.
> 

But __GFP_NOWARN allocation (like in this case) should just fail silently
without warnings regardless of reason because caller can deal with that.

Without __GFP_NOWARN allocator should print standard warning.

Caller anyway must handle NULL\ENOMEM result - this error path
should be used for handling impossible sizes too.
Of course it could check size first, just as optimization.

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-01  9:12 [PATCH] mm/kvmalloc: do not confuse kmalloc with page order over MAX_ORDER Konstantin Khlebnikov
2018-11-01  9:33 ` Michal Hocko
2018-11-01 10:09 ` [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE Konstantin Khlebnikov
2018-11-01 10:24   ` Michal Hocko
2018-11-01 10:48     ` Konstantin Khlebnikov
2018-11-01 12:55       ` Michal Hocko
2018-11-01 16:42         ` Konstantin Khlebnikov [this message]
2018-11-01 16:55           ` Michal Hocko
2018-11-05 13:03   ` Vlastimil Babka
2018-11-05 16:19     ` Konstantin Khlebnikov
2018-11-05 16:52       ` Vlastimil Babka
2018-11-05 16:57       ` Michal Hocko

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=ae51e16b-459c-7d59-6277-b1a197dbf5ff@yandex-team.ru \
    --to=khlebnikov@yandex-team.ru \
    --cc=akpm@linux-foundation.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-mm Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-mm/0 linux-mm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-mm linux-mm/ https://lore.kernel.org/linux-mm \
		linux-mm@kvack.org
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kvack.linux-mm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git