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From: Andrew Morton <akpm@linux-foundation.org>
To: Song Liu <songliubraving@fb.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	"matthew.wilcox@oracle.com" <matthew.wilcox@oracle.com>,
	"kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>,
	Kernel Team <Kernel-team@fb.com>,
	"william.kucharski@oracle.com" <william.kucharski@oracle.com>,
	"chad.mynhier@oracle.com" <chad.mynhier@oracle.com>,
	"mike.kravetz@oracle.com" <mike.kravetz@oracle.com>
Subject: Re: [PATCH v2 0/3] Enable THP for text section of non-shmem files
Date: Wed, 19 Jun 2019 18:13:54 -0700
Message-ID: <20190619181354.325242c09d5c2ef44f430b4a@linux-foundation.org> (raw)
In-Reply-To: <BA4D64DA-4F48-4683-8512-0402B9533EE7@fb.com>

On Tue, 18 Jun 2019 21:48:16 +0000 Song Liu <songliubraving@fb.com> wrote:

> > I'm wondering if this limitation can be abused in some fashion: mmap a
> > file to which you have read permissions, run madvise(MADV_HUGEPAGE) and
> > thus prevent the file's owner from being able to modify the file?  Or
> > something like that.  What are the issues and protections here?
> 
> In this case, the owner need to make a copy of the file, and then remove 
> and update the original file. 
> 
> In this version, we want either split huge page on writes, or fail the 
> write when we cannot split. However, the huge page information is only 
> available at page level, and on the write path, page level information 
> is not available until write_begin(). So it is hard to stop writes at 
> earlier stage. Therefore, in this version, we leverage i_mmap_writable, 
> which is at address_space level. So it is easier to stop writes to the 
> file. 
> 
> This is a temporary behavior. And it is gated by the config. So I guess
> it is OK. It works well for our use cases though. Once we have better 
> write support, we can remove the limitation. 
> 
> If this is too weird, I am also open to suggestions. 

Well, it's more than weird?  This permits user A to deny service to
user B?  User A can, maliciously or accidentally, prevent user B from
modifying a file which user B has permission to modify?  Such as, umm,
/etc/hosts?


  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-14 18:22 Song Liu
2019-06-14 18:22 ` [PATCH v2 1/3] mm: check compound_head(page)->mapping in filemap_fault() Song Liu
2019-06-17 14:59   ` Rik van Riel
2019-06-14 18:22 ` [PATCH v2 2/3] mm,thp: stats for file backed THP Song Liu
2019-06-17 15:00   ` Rik van Riel
2019-06-21 12:50   ` Kirill A. Shutemov
2019-06-21 14:09     ` Song Liu
2019-06-14 18:22 ` [PATCH v2 3/3] mm,thp: add read-only THP support for (non-shmem) FS Song Liu
2019-06-17 15:42   ` Rik van Riel
2019-06-21 12:58   ` Kirill A. Shutemov
2019-06-21 13:08     ` Song Liu
2019-06-21 13:11       ` Kirill A. Shutemov
2019-06-18 21:12 ` [PATCH v2 0/3] Enable THP for text section of non-shmem files Andrew Morton
2019-06-18 21:48   ` Song Liu
2019-06-20  1:13     ` Andrew Morton [this message]
2019-06-20  2:04       ` Song Liu
2019-06-19  6:26   ` Song Liu

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