linux-kernel - Re: [PATCH v4 3/4] mm/memory_hotplug: Define nodemask

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Message-ID: <cc7e1bac-2efb-b23f-bd76-8e836abeba0e@microsoft.com>
Date:   Tue, 28 Aug 2018 14:04:18 +0000
From:   Pasha Tatashin <Pavel.Tatashin@...rosoft.com>
To:     Oscar Salvador <osalvador@...hadventures.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "mhocko@...e.com" <mhocko@...e.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "yasu.isimatu@...il.com" <yasu.isimatu@...il.com>,
        "jonathan.cameron@...wei.com" <jonathan.cameron@...wei.com>,
        "david@...hat.com" <david@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v4 3/4] mm/memory_hotplug: Define nodemask_t as a stack
 variable



On 8/17/18 5:00 AM, Oscar Salvador wrote:
> From: Oscar Salvador <osalvador@...e.de>
> 
> Currently, unregister_mem_sect_under_nodes() tries to allocate a nodemask_t
> in order to check whithin the loop which nodes have already been unlinked,
> so we do not repeat the operation on them.
> 
> NODEMASK_ALLOC calls kmalloc() if NODES_SHIFT > 8, otherwise
> it just declares a nodemask_t variable whithin the stack.
> 
> Since kmalloc() can fail, we actually check whether NODEMASK_ALLOC failed
> or not, and we return -ENOMEM accordingly.
> remove_memory_section() does not check for the return value though.
> It is pretty rare that such a tiny allocation can fail, but if it does,
> we will be left with dangled symlinks under /sys/devices/system/node/,
> since the mem_blk's directories will be removed no matter what
> unregister_mem_sect_under_nodes() returns.
> 
> One way to solve this is to check whether unlinked_nodes is NULL or not.
> In the case it is not, we can just use it as before, but if it is NULL,
> we can just skip the node_test_and_set check, and call sysfs_remove_link()
> unconditionally.
> This is harmless as sysfs_remove_link() backs off somewhere down the chain
> in case the link has already been removed.
> This method was presented in v3 of the path [1].
> 
> But since the maximum number of nodes we can have is 1024,
> when NODES_SHIFT = 10, that gives us a nodemask_t of 128 bytes.
> Although everything depends on how deep the stack is, I think we can afford
> to define the nodemask_t variable whithin the stack.
> 
> That simplifies the code, and we do not need to worry about untested error
> code paths.
> 
> If we see that this causes troubles with the stack, we can always return to [1].
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>

LGTM:

Reviewed-by: Pavel Tatashin <pavel.tatashin@...rosoft.com>

Thank you,
Pavel