On Wed 30-01-19 13:44:18, Vlastimil Babka wrote:
> From: Jiri Kosina <[email protected]>
>
> The semantics of what mincore() considers to be resident is not completely
> clear, but Linux has always (since 2.3.52, which is when mincore() was
> initially done) treated it as "page is available in page cache".
>
> That's potentially a problem, as that [in]directly exposes meta-information
> about pagecache / memory mapping state even about memory not strictly
> belonging
> to the process executing the syscall, opening possibilities for sidechannel
> attacks.
>
> Change the semantics of mincore() so that it only reveals pagecache
> information
> for non-anonymous mappings that belog to files that the calling process could
> (if it tried to) successfully open for writing.
I agree that this is a better way than the original 574823bfab82
("Change mincore() to count "mapped" pages rather than "cached" pages").
One thing is still not clear to me though. Is the new owner/writeable
check OK for the Netflix-like usecases? I mean does happycache have
appropriate access to the cache data? I have tried to re-read the
original thread but couldn't find any confirmation.
I nit below
> Originally-by: Linus Torvalds <[email protected]>
> Originally-by: Dominique Martinet <[email protected]>
> Cc: Dominique Martinet <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Cc: Dave Chinner <[email protected]>
> Cc: Kevin Easton <[email protected]>
> Cc: Matthew Wilcox <[email protected]>
> Cc: Cyril Hrubis <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Cc: Kirill A. Shutemov <[email protected]>
> Cc: Daniel Gruss <[email protected]>
> Signed-off-by: Jiri Kosina <[email protected]>
> Signed-off-by: Vlastimil Babka <[email protected]>
other than that looks good to me.
Acked-by: Michal Hocko <[email protected]>
If this still doesn't help happycache kind of workloads then we should
add a capability check IMO but this looks like a decent foundation to
me.
> ---
> mm/mincore.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 218099b5ed31..747a4907a3ac 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -169,6 +169,14 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long
> addr, unsigned long end,
> return 0;
> }
>
> +static inline bool can_do_mincore(struct vm_area_struct *vma)
> +{
> + return vma_is_anonymous(vma) ||
> + (vma->vm_file &&
> + (inode_owner_or_capable(file_inode(vma->vm_file))
> + || inode_permission(file_inode(vma->vm_file),
> MAY_WRITE) == 0));
> +}
This is hard to read. Can we do
if (vma_is_anonymous(vma))
return true;
if (!vma->vm_file)
return false;
return inode_owner_or_capable(file_inode(vma->vm_file)) ||
inode_permission(file_inode(vma->vm_file), MAY_WRITE) == 0;
--
Michal Hocko
SUSE Labs
