linux-kernel - Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

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Message-ID: <982317c0-7faa-45f0-82a1-29978c3c9f4d@intel.com>
Date: Fri, 24 May 2024 10:16:39 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Byungchul Park <byungchul@...com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: kernel_team@...ynix.com, akpm@...ux-foundation.org, ying.huang@...el.com,
 vernhao@...cent.com, mgorman@...hsingularity.net, hughd@...gle.com,
 willy@...radead.org, david@...hat.com, peterz@...radead.org,
 luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, rjgolo@...il.com
Subject: Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over
 90%

On 5/9/24 23:51, Byungchul Park wrote:
> To achieve that:
> 
>    1. For the folios that map only to non-writable tlb entries, prevent
>       tlb flush during unmapping but perform it just before the folios
>       actually become used, out of buddy or pcp.

Is this just _pure_ unmapping (like MADV_DONTNEED), or does it apply to
changing the memory map, like munmap() itself?

>    2. When any non-writable ptes change to writable e.g. through fault
>       handler, give up luf mechanism and perform tlb flush required
>       right away.
> 
>    3. When a writable mapping is created e.g. through mmap(), give up
>       luf mechanism and perform tlb flush required right away.

Let's say you do this:

	fd = open("/some/file", O_RDONLY);
	ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
	foo1 = *ptr1;

You now have a read-only PTE pointing to the first page of /some/file.
Let's say try_to_unmap() comes along and decides it can_luf_folio().
The page gets pulled out of the page cache and freed, the PTE is zeroed.
 But the TLB is never flushed.

Now, someone does:

	fd2 = open("/some/other/file", O_RDONLY);
	ptr2 = mmap(ptr1, size, PROT_READ, MAP_FIXED, fd, ...);
	foo2 = *ptr2;

and they overwrite the old VMA.  Does foo2 have the contents of the new
"/some/other/file" or the old "/some/file"?  How does the new mmap()
know that there was something to flush?

BTW, the same thing could happen without a new mmap().  Someone could
modify the file in the middle, maybe even from another process.

	fd = open("/some/file", O_RDONLY);
	ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
	foo1 = *ptr1;
	// LUF happens here
	// "/some/file" changes
	foo2 = *ptr1; // Does this see the change?