linux-kernel - Re: [PATCH 13/31] mm/hmm: retry if pte_offset

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Message-ID: <0e3c5937-2f04-25d9-98eb-01e4c2b04150@linux.dev>
Date:   Mon, 22 May 2023 20:11:36 +0800
From:   Qi Zheng <qi.zheng@...ux.dev>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 13/31] mm/hmm: retry if pte_offset_map() fails



On 2023/5/22 13:05, Hugh Dickins wrote:
> hmm_vma_walk_pmd() is called through mm_walk, but already has a goto
> again loop of its own, so take part in that if pte_offset_map() fails.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>   mm/hmm.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index e23043345615..b1a9159d7c92 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -381,6 +381,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>   	}
>   
>   	ptep = pte_offset_map(pmdp, addr);
> +	if (!ptep)
> +		goto again;
>   	for (; addr < end; addr += PAGE_SIZE, ptep++, hmm_pfns++) {
>   		int r;
>   

I haven't read the entire patch set yet, but taking a note here.
The hmm_vma_handle_pte() will unmap pte and then call
migration_entry_wait() to remap pte, so this may fail, we need to
handle this case like below:

diff --git a/mm/hmm.c b/mm/hmm.c
index 6a151c09de5e..eb726ff0981c 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -276,7 +276,8 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, 
unsigned long addr,
                 if (is_migration_entry(entry)) {
                         pte_unmap(ptep);
                         hmm_vma_walk->last = addr;
-                       migration_entry_wait(walk->mm, pmdp, addr);
+                       if (!migration_entry_wait(walk->mm, pmdp, addr))
+                               return -EAGAIN;
                         return -EBUSY;
                 }

@@ -386,6 +387,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,

                 r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, 
hmm_pfns);
                 if (r) {
+                       if (r == -EAGAIN)
+                               goto again;
                         /* hmm_vma_handle_pte() did pte_unmap() */
                         return r;
                 }

Of course, the migration_entry_wait() also needs to be modified.

-- 
Thanks,
Qi