linux-kernel - Re: [PATCH v2] fs/proc/page: Skip uninitialized page when iterating page structures

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Date:   Mon, 26 Aug 2019 09:49:38 -0400
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Stephen Rothwell <sfr@...b.auug.org.au>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2] fs/proc/page: Skip uninitialized page when iterating
 page structures

On 8/26/19 9:43 AM, Waiman Long wrote:
> On 8/26/19 9:25 AM, Matthew Wilcox wrote:
>>
>> Would this not work equally well?
>>
>> +++ b/fs/proc/page.c
>> @@ -46,7 +46,8 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>>                         ppage = pfn_to_page(pfn);
>>                 else
>>                         ppage = NULL;
>> -               if (!ppage || PageSlab(ppage) || page_has_type(ppage))
>> +               if (!ppage || PageSlab(ppage) || page_has_type(ppage) ||
>> +                               PagePoisoned(ppage))
>>                         pcount = 0;
>>                 else
>>                         pcount = page_mapcount(ppage);
>>
> That is my initial thought too. However, I couldn't find out where the
> memory of the uninitialized page structures may have been initialized
> somehow. The only thing I found is when vm_debug is on that the page
> structures are indeed poisoned. Without that it is probably just
> whatever the content that the memory have when booting up the kernel.
>
> It just happens on the test system that I used the memory of those page
> structures turned out to be -1. It may be different in other systems
> that can still crash the kernel, but not detected by the PagePoisoned()
> check. That is why I settle on the current scheme which is more general
> and don't rely on the memory get initialized in a certain way.

Actually, I have also thought about always poisoning the page
structures. However, that will introduce additional delay in the boot up
process which can be problematic especially if the system has large
amount of persistent memory.

Cheers,
Longman