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On 6 Feb 2026, at 13:29, Zi Yan wrote: > On 6 Feb 2026, at 13:21, Mikhail Gavrilov wrote: > >> Hi, Yan >> >> On Fri, Feb 6, 2026 at 11:08=E2=80=AFPM Zi Yan wrote: >>> >>> Do you have a reproducer for this issue? >> >> Yes, I have a stress test that reliably reproduces the crash. >> It cycles swapon/swapoff on 8GB zram under memory pressure: >> https://gist.github.com/NTMan/4ed363793ebd36bd702a39283f06cee1 Got it. Merging replies from Kairui from another email: This patch is from previous discussion: https://lore.kernel.org/linux-mm/CABXGCsO3XcXt5GDae7d74ynC6P6G2gLw3ZrwAYvSQ= 3PwP0mGXA@mail.gmail.com/ It looks odd to me too. That bug starts with vmalloc dropping __GFP_COMP in commit 3b8000ae185c, because with __GFP_COMP, the allocator does clean the ->private of tail pages on allocation with prep_compound_page. Without __GFP_COMP, these ->private fields are left as it is. >> >>> Last time I checked page->private usage, I find users clears ->private = before free a page. >>> I wonder which one I was missing. >> >> The issue is not about freeing - it's about allocation. I assume everyone zeros used ->private, head or tail, so PageBuddy has all zeroed ->private. >> When buddy allocator merges/splits pages, it uses page->private to store= order. >> When a high-order page is later allocated and split via split_page(), >> tail pages still have their old page->private values. No, in __free_one_page(), if a free page is merged to a higher order, it is deleted from free list and its ->private is zeroed. There should not be any non zero private. >> The path is: >> 1. Page freed =E2=86=92 free_pages_prepare() does NOT clear page->privat= e Right. The code assume page->private is zero for all pages, head or tail if it is compound. >> 2. Page goes to buddy allocator =E2=86=92 buddy uses page->private for o= rder >> 3. Page allocated as high-order =E2=86=92 post_alloc_hook() only clears = head >> page's private >> 4. split_page() called =E2=86=92 tail pages keep stale page->private >> >>> Clearing ->private in split_page() looks like a hack instead of a fix. >> >> I discussed this with Kairui Song earlier in the thread. We considered: >> >> 1. Fix in post_alloc_hook() - would need to clear all pages, not just he= ad >> 2. Fix in swapfile.c - doesn't work because stale value could >> accidentally equal SWP_CONTINUED >> 3. Fix in split_page() - ensures pages are properly initialized for >> independent use >> >> The comment in vmalloc.c says split pages should be usable >> independently ("some use page->mapping, page->lru, etc."), so >> split_page() initializing the pages seems appropriate. >> >> But I agree post_alloc_hook() might be a cleaner place. Would you >> prefer a patch there instead? I think it is better to find out which code causes non zero ->private at page free time. Best Regards, Yan, Zi