linux-kernel - Re: [PATCH] mm: ksm: fix data-race in __ksm_enter / run

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Message-ID: <YulGbmJt2faelV6c@casper.infradead.org>
Date:   Tue, 2 Aug 2022 16:44:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Abhishek Shah <abhishek.shah@...umbia.edu>,
        Gabriel Ryan <gabe@...columbia.edu>
Subject: Re: [PATCH] mm: ksm: fix data-race in __ksm_enter / run_store

On Tue, Aug 02, 2022 at 11:15:50PM +0800, Kefeng Wang wrote:
> The ksm_run is alread protected by ksm_thread_mutex in run_store, we
> could add this lock in __ksm_enter() to avoid the above issue.

I don't think this is a great fix.  Why not protect the store with
ksm_mmlist_lock?  ie:

        mutex_lock(&ksm_thread_mutex);
        wait_while_offlining();
        if (ksm_run != flags) {
+		spin_lock(&ksm_mmlist_lock);
                ksm_run = flags;
+		spin_unlock(&ksm_mmlist_lock);
                if (flags & KSM_RUN_UNMERGE) {
                        set_current_oom_origin();
                        err = unmerge_and_remove_all_rmap_items();
                        clear_current_oom_origin();
                        if (err) {
+				spin_lock(&ksm_mmlist_lock);
				ksm_run = KSM_RUN_STOP;
+				spin_unlock(&ksm_mmlist_lock);
...

(I also don't think this is a real bug, because the call to
unmerge_and_remove_all_rmap_items() will "cure" the misplacement of
items in the list, but there's value in shutting up the tools, I suppose)