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Date:   Tue, 10 Jan 2023 17:05:06 +0200
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Subject: Re: [PATCH] lib/scatterlist: Fix to calculate the last_pg properly
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To:     Jason Gunthorpe <jgg@nvidia.com>
CC:     Matthew Wilcox <willy@infradead.org>,
        <linux-kernel@vger.kernel.org>, <linux-block@vger.kernel.org>,
        <linux-mm@kvack.org>, <axboe@kernel.dk>, <logang@deltatee.com>,
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References: <20230109144701.83021-1-yishaih@nvidia.com>
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From:   Yishai Hadas <yishaih@nvidia.com>
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On 10/01/2023 15:53, Jason Gunthorpe wrote:
> On Tue, Jan 10, 2023 at 10:01:53AM +0200, Yishai Hadas wrote:
>> On 10/01/2023 7:08, Matthew Wilcox wrote:
>>> On Mon, Jan 09, 2023 at 04:47:01PM +0200, Yishai Hadas wrote:
>>>>    	if (sgt_append->prv) {
>>>> +		unsigned long paddr =
>>>> +			(page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
>>> 1. page_to_pfn() * PAGE_SIZE is spelled page_to_phys()
>>>
>>> 2. physical addresses have type phys_addr_t.  Oh, wait, paddr isn't a
>>> physical address, it's a pfn because you divide by PAGE_SIZE at the end.
>>> But you will get truncation on 32-bit, because page_to_pfn() has type
>>> unsigned long.  Anyway, this shouldn't be called paddr.  Maybe last_pfn
>>> or something?
>>>
>>>> +			 sgt_append->prv->offset + sgt_append->prv->length) /
>>>> +			PAGE_SIZE;
>> In this area of the patch I just reverted to what we had in 6.1 [1] where we
>> were good.
>>
>> I can send V1 and replace the 'paddr' to be called 'last_pfn' if it's really
>> needed.
> Please change it as Matthew describes so it is clearer
>
> Jason

OK

Once cleaning the code and renaming to 'last_pfn', could also see that 
we had to use below '- 1' instead of '- PAGE_SIZE'.

The expected diff compared to V0 can be seen below.

Will repeat some testing and send V1 once be ready.

+++ b/lib/scatterlist.c
@@ -470,18 +470,16 @@ int sg_alloc_append_table_from_pages(struct 
sg_append_table *sgt_append,
                 return -EOPNOTSUPP;

         if (sgt_append->prv) {
-               unsigned long paddr =
-                       (page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
-                        sgt_append->prv->offset + 
sgt_append->prv->length) /
-                       PAGE_SIZE;
+               unsigned long last_pfn = 
(page_to_phys(sg_page(sgt_append->prv)) +
+                       sgt_append->prv->offset + 
sgt_append->prv->length) / PAGE_SIZE;

                 if (WARN_ON(offset))
                         return -EINVAL;

                 /* Merge contiguous pages into the last SG */
                 prv_len = sgt_append->prv->length;
-               if (page_to_pfn(pages[0]) == paddr) {
-                       last_pg = pfn_to_page(paddr - PAGE_SIZE);
+               if (page_to_pfn(pages[0]) == last_pfn) {
+                       last_pg = pfn_to_page(last_pfn - 1);
                         while (n_pages && pages_are_mergeable(pages[0], 
last_pg)) {
                                 if (sgt_append->prv->length + PAGE_SIZE 
 > max_segment)
                                         break;

Yishai