linux-kernel - Re: [PATCH] mm/compaction: fix the total

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Message-ID:
	<TYSPR01MB5702BA8F9AE17746F198F077F0592@TYSPR01MB5702.apcprd01.prod.exchangelabs.com>
Date: Tue, 12 Nov 2024 07:51:53 +0000
From: "liuq131@...natelecom.cn" <liuq131@...natelecom.cn>
To: "baolin.wang@...ux.alibaba.com" <baolin.wang@...ux.alibaba.com>
CC: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "liuq131@...natelecom.cn"
	<liuq131@...natelecom.cn>
Subject: Re: [PATCH] mm/compaction: fix the total_isolated in strict mode

We assume that the block we are currently processing is distributed as follows:
0   1   2                                                            511
--------------------------------------------------
|    |    |                                                              |
--------------------------------------------------
Index 0 and 1 are both pages with an order of 0.
Index 2 has a bogus order (let's assume the order is 9).
When the for loop reaches index 2, it will enter the following code:
/*
* For compound pages such as THP and hugetlbfs, we can save
* potentially a lot of iterations if we skip them at once.
* The check is racy, but we can consider only valid values
* and the only danger is skipping too much.
*/
if (PageCompound(page)) {
    const unsigned int order = compound_order(page);
    if (blockpfn + (1UL << order) <= end_pfn) {
        blockpfn += (1UL << order) - 1;
        page += (1UL << order) - 1;
        nr_scanned += (1UL << order) - 1;
    }
    goto isolate_fail;

}

After exiting the for loop:
blockpfn =basepfn+ 2+2^9 = basepfn+514;
endpfn  = basepfn +512;
total_isolated = 2;
nr_scanned = 514;

/*
* Be careful to not go outside of the pageblock.
*/
if (unlikely(blockpfn > end_pfn))
        blockpfn = end_pfn;
So this can happen

/*
* If strict isolation is requested by CMA then check that all the
* pages requested were isolated. If there were any failures, 0 is
* returned and CMA will fail.
*/
if (strict && blockpfn < end_pfn)
        total_isolated = 0;

If processed according to the old code, it will not enter the if statement to reset 
total_isolated, but the correct handling is to reset total_isolated to 0.