Re: Race between vmtruncate and mapped areas?

[Mika Penttilä]

Isn't that what inode->i_sem is supposed to protect...?

--Mika

Dave McCracken wrote:

>As part of chasing the BUG() we've been seeing in objrmap I took a good
>look at vmtruncate().  I believe I've identified a race condition that no
>only  triggers that BUG(), but also could cause some strange behavior
>without the objrmap patch.
>
>Basically vmtruncate() does the following steps:  first, it unmaps the
>truncated pages from all page tables using zap_page_range().  Then it
>removes those pages from the page cache using truncate_inode_pages().
>These steps are done without any lock that I can find, so it's possible for
>another task to get in between the unmap and the remove, and remap one or
>more pages back into its page tables.
>
>The result of this is a page that has been disconnected from the file but
>is mapped in a task's address space as if it were still part of that file.
>Any further modifications to this page will be lost.
>
>I can easily detect this condition by adding a bugcheck for page_mapped()
>in truncate_complete_page(), then running Andrew's bash-shared-mapping test
>case.
>
>Please feel free to poke holes in my analysis.  I'm not at all sure I
>haven't missed some subtlety here.
>
>Dave McCracken
>
>======================================================================
>Dave McCracken          IBM Linux Base Kernel Team      1-512-838-3059
>[e-mail]                                        T/L   678-3059
>
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