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Message-ID: <0e3c5937-2f04-25d9-98eb-01e4c2b04150@linux.dev>
Date:   Mon, 22 May 2023 20:11:36 +0800
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Subject: Re: [PATCH 13/31] mm/hmm: retry if pte_offset_map() fails
Content-Language: en-US
To:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com>
 <2edc4657-b6ff-3d6e-2342-6b60bfccc5b@google.com>
From:   Qi Zheng <qi.zheng@linux.dev>
In-Reply-To: <2edc4657-b6ff-3d6e-2342-6b60bfccc5b@google.com>
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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@google.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