Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

[Song Liu]

On Fri, Sep 24, 2021 at 12:12 AM Rongwei Wang
<rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 9/24/21 10:43 AM, Andrew Morton wrote:
> > On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang <rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
> >
> >>
> >>
> >>> On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >>>
> >>> On Wed, Sep 22, 2021 at 03:06:44PM +0800, Rongwei Wang wrote:
> >>>> Transparent huge page has supported read-only non-shmem files. The file-
> >>>> backed THP is collapsed by khugepaged and truncated when written (for
> >>>> shared libraries).
> >>>>
> >>>> However, there is race in two possible places.
> >>>>
> >>>> 1) multiple writers truncate the same page cache concurrently;
> >>>> 2) collapse_file rolls back when writer truncates the page cache;
> >>>
> >>> As I've said before, the bug here is that somehow there is a writable fd
> >>> to a file with THPs.  That's what we need to track down and fix.
> >> Hi, Matthew
> >> I am not sure get your means. We know “mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs"
> >> Introduced file-backed THPs for DSO. It is possible {very rarely} for DSO to be opened in writeable way.
> >>
> >> ...
> >>
> >>> https://lore.kernel.org/linux-mm/YUdL3lFLFHzC80Wt@xxxxxxxxxxxxxxxxxxxx/
> >> All in all, what you mean is that we should solve this race at the source?
> >
> > Matthew is being pretty clear here: we shouldn't be permitting
> > userspace to get a writeable fd for a thp-backed file.
> >
> > Why are we permitting the DSO to be opened writeably?  If there's a
> > legitimate case for doing this then presumably "mm, thp: relax the
> There is a use case to stress file-backed THP within attachment.
> I test this case in a system which has enabled CONFIG_READ_ONLY_THP_FOR_FS:
>
> $ gcc -Wall -g -o stress_madvise_dso stress_madvise_dso.c
> $ ulimit -s unlimited
> $ ./stress_madvise_dso 10000 <libtest.so>
>
> the meaning of above parameters:
> 10000: the max test time;
> <libtest.so>: the DSO that will been mapped into file-backed THP by
> madvise. It recommended that the text segment of DSO to be tested is
> greater than 2M.
>
> The crash will been triggered at once in the latest kernel. And this
> case also can used to trigger the bug that mentioned in our another patch.

Hmm.. I am not able to use the repro program to crash the system. Not
sure what I did wrong.

OTOH, does it make sense to block writes within khugepaged, like:

diff --git i/mm/khugepaged.c w/mm/khugepaged.c
index 045cc579f724e..ad7c41ec15027 100644
--- i/mm/khugepaged.c
+++ w/mm/khugepaged.c
@@ -51,6 +51,7 @@ enum scan_result {
        SCAN_CGROUP_CHARGE_FAIL,
        SCAN_TRUNCATED,
        SCAN_PAGE_HAS_PRIVATE,
+       SCAN_BUSY_WRITE,
 };

 #define CREATE_TRACE_POINTS
@@ -1652,6 +1653,11 @@ static void collapse_file(struct mm_struct *mm,
        /* Only allocate from the target node */
        gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;

+       if (deny_write_access(file)) {
+               result = SCAN_BUSY_WRITE;
+               return;
+       }
+
        new_page = khugepaged_alloc_page(hpage, gfp, node);
        if (!new_page) {
                result = SCAN_ALLOC_HUGE_PAGE_FAIL;
@@ -1863,19 +1869,6 @@ static void collapse_file(struct mm_struct *mm,
        else {
                __mod_lruvec_page_state(new_page, NR_FILE_THPS, nr);
                filemap_nr_thps_inc(mapping);
-               /*
-                * Paired with smp_mb() in do_dentry_open() to ensure
-                * i_writecount is up to date and the update to nr_thps is
-                * visible. Ensures the page cache will be truncated if the
-                * file is opened writable.
-                */
-               smp_mb();
-               if (inode_is_open_for_write(mapping->host)) {
-                       result = SCAN_FAIL;
-                       __mod_lruvec_page_state(new_page, NR_FILE_THPS, -nr);
-                       filemap_nr_thps_dec(mapping);
-                       goto xa_locked;
-               }
        }

        if (nr_none) {
@@ -1976,6 +1969,7 @@ static void collapse_file(struct mm_struct *mm,
        VM_BUG_ON(!list_empty(&pagelist));
        if (!IS_ERR_OR_NULL(*hpage))
                mem_cgroup_uncharge(*hpage);
+       allow_write_access(file);
        /* TODO: tracepoints */
 }