linux-kernel - Re: [PATCH] mm/memory-failure: remove shake

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Date: Fri, 26 Apr 2024 13:33:27 -0700
From: Jane Chu <jane.chu@...cle.com>
To: Matthew Wilcox <willy@...radead.org>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, linmiaohe@...wei.com,
        nao.horiguchi@...il.com, osalvador@...e.de
Subject: Re: [PATCH] mm/memory-failure: remove shake_page()

My apology for the gobbled message earlier.

On 4/26/2024 12:52 PM, Jane Chu wrote:
> On 4/26/2024 12:05 PM, Matthew Wilcox wrote:
> [..]
>> That would be unsafe, the safe way would be if we moved page_folio() 
>> after
>>> the call to __get_hw_poison() in get_any_page() and there would 
>>> still be one
>>> remaining user of shake_page() that we can't convert. A safe version 
>>> of this
>>> patch would result in a removal of one use of PageHuge() and two 
>>> uses of
>>> put_page(), would that be worth submitting?
>>>
>>> get_any_page()
>>>     if(__get_hwpoison_page())
>>>         folio = page_folio() /* folio_try_get() returned 1, safe */
>> I think we should convert __get_hwpoison_page() to return either the 
>> folio
>> or an ERR_PTR or NULL.  Also, I think we should delete the "cannot catch
>> tail" part and just loop in __get_hwpoison_page() until we do catch it.
>> See try_get_folio() in mm/gup.c for inspiration (although you can't use
>> it exactly because that code knows that the page is mapped into a page
>> table, so has a refcount).
>>
>> But that's just an immediate assessment; you might find a reason that
>> doesn't work.
>
Besides, in a possible hugetlb demote scenario, it seems to me that we 
should retry
get_hwpoison_hugetlb_folio() instead of falling thru to folio_try_get().

static int __get_hwpoison_page(struct page *page, unsigned long flags)
{
         struct folio *folio = page_folio(page);
         int ret = 0;
         bool hugetlb = false;

         ret = get_hwpoison_hugetlb_folio(folio, &hugetlb, false);
         if (hugetlb) {
                 /* Make sure hugetlb demotion did not happen from under 
us. */
                 if (folio == page_folio(page))
                         return ret;
                 if (ret > 0) {      <===== still hugetlb, don't fall 
thru, retry
                         folio_put(folio);
                         folio = page_folio(page);
                 }

         }

thanks!
-jane