linux-kernel - Re: [PATCH 1/3] selftests/mm: virtual_address_range: Fix error when CommitLimit

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Message-ID: <4e52d67c-e968-4cf6-9c9b-88646f0d3a23@redhat.com>
Date: Thu, 9 Jan 2025 14:05:43 +0100
From: David Hildenbrand <david@...hat.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Dev Jain <dev.jain@....com>, Andrew Morton <akpm@...ux-foundation.org>,
 Shuah Khan <shuah@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH 1/3] selftests/mm: virtual_address_range: Fix error when
 CommitLimit < 1GiB

 >
> That is clear. The issue would be to figure which chunks are valid to
> unmap. If something critical like the executable file is unmapped,
> the process crashes. But see below.

Ah, now I see what you mean. Yes, also the stack etc. will be 
problematic. So IIUC, you want to limit the munmap optimization only to 
the manually mmap()ed parts.

> 
>>> Is it fine to rely on CONFIG_ANON_VMA_NAME?
>>> That would make it much easier to implement.
>>
>> Can you elaborate how you would do it?
> 
> First set the VMA name after mmap():
> 
> for (i = 0; i < NR_CHUNKS_LOW; i++) {
> 	ptr[i] = mmap(NULL, MAP_CHUNK_SIZE, PROT_READ | PROT_WRITE,
> 		     MAP_NORESERVE | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> 
> 	if (ptr[i] == MAP_FAILED) {
> 		if (validate_lower_address_hint())
> 			ksft_exit_fail_msg("mmap unexpectedly succeeded with hint\n");
> 		break;
> 	}
> 
> 	validate_addr(ptr[i], 0);
> 	if (prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, ptr[i], MAP_CHUNK_SIZE, "virtual_address_range"))
> 		ksft_exit_fail_msg("prctl(PR_SET_VMA_ANON_NAME) failed: %s\n", strerror(errno));

Likely this would prevent merging of VMAs.

With a 1 GiB chunk size, and NR_CHUNKS_LOW == 128TiB, you'd already 
require 128k VMAs. The default limit is frequently 64k.

We could just scan the ptr / hptr array to see if this is a manual mmap 
area or not. If this takes too long, one could sort the arrays by 
address and perform a binary search.

Not the most efficient way of doing it, but maybe good enough for this test?

Alternatively, store the pointer in a xarray-like tree instead of two 
arrays. Requires a bit more memory ... and we'd have to find a simple 
implementation we could just reuse in this test. So maybe there is a 
simpler way to get it done.

-- 
Cheers,

David / dhildenb