(adding Vishal) On Wed, May 20, 2026 at 01:59:49AM +0900, Juhyung Park wrote: > On Wed, May 20, 2026 at 1:41 AM Dave Hansen <[email protected]> wrote: > > > > On 5/19/26 09:27, Juhyung Park wrote: > > > Hi Dave, > > > > > > On Wed, May 20, 2026 at 1:02 AM Dave Hansen <[email protected]> wrote: > > >> > > >> On 5/19/26 08:10, Juhyung Park wrote: > > >>> #endif > > >>> } else { > > >>> - pagetable_free(page_ptdesc(page)); > > >>> + /* > > >>> + * Use __free_pages() to honor @order: vmemmap PMD leaves > > >>> + * freed here are not compound pages, so pagetable_free() > > >>> + * would lose leak 511 of 512 pages per 2 MB chunk. > > >>> + */ > > >>> + __free_pages(page, order); > > >>> } > > >>> } > > >> > > >> I find myself really wondering how much of this came from a human and > > >> how much from the LLM. Could you share that with us? > > > > > > Not my first kernel contribution, just so you know. (first in mm tho) > > > > > > I asked Claude to write both the commit body and comment and it was > > > too verbose. I manually trimmed it down. > > > Sorry if it still sounds too LLM-ish. > > > > Yeah, it still sounded really LLM-ish to me. Still rather chatty. > > > > > This was tested on a VM with virtualized CXL device and toggling it > > > back and forth was visibly causing leaks. kmemleak was unable to catch > > > this (rightfully so), so I skeptically asked Claude to see if it can > > > figure it out while pwd was the kernel source the VM was running. > > > "Access the VM at "ssh -p2223 [email protected]". There's a memory > > > leak whenever CXL memory switches modes via: daxctl reconfigure-device > > > --mode=system-ram dax0.0 --force, daxctl reconfigure-device > > > --mode=devdax dax0.0 --force. Figure out why. If you need to reboot > > > the VM, do not do it yourself and ask me." > > > > > > It did in 6 minutes and it basically told me to revert bf9e4e30f353. I > > > was very skeptical and reviewed manually (with my short knowledge of > > > mm) why this would be a correct fix. > > > > Neato. > > > > >> We're trying to get _away_ from using the 'struct page' APIs on page > > >> tables. This goes backwards. Worst case, do: > > >> > > >> /* vmemmap PMD leaves are not compound pages */ > > >> for (i = 0; i < 1<<order; i++) > > >> pagetable_free(page_ptdesc(&page[i])); > > >> > > >> Right? > > > > > > Shouldn't I worry about the loop overhead? With order == 9, that's 512 > > > iterations. That's compounded to O(N) when the entire memory size is > > > in consideration. > > > > Is it optimal? No. > > > > Will anybody ever notice? Also no. > > > > Will anybody ever care? No sir. > > Just spun a test with that loop. It doesn't fix the leak. > > I hate to be the guy that copy-pastas LLM but this is outside my > knowledge of mm. Claude suggests: > "Each pagetable_free() on the tails is a no-op: When > alloc_pages_node(node, gfp, order=9) returns without __GFP_COMP, the > buddy allocator only sets _refcount = 1 on the head page. The other > 511 pages (page[1] … page[511]) have _refcount = 0. There's no > compound metadata, so they aren't "tails" in the folio sense either — > they're just contiguous pages whose refcounts the allocator never > touched." > > Any ideas? > > Thanks. > > > > > Can you measure the difference? I'd wager a beer: No again. > > > > Even if someone manages to notice, then you have a clear path to fix it > > *right*: fix the ptdesc data structure to represent high-order allocations.
-- Sincerely yours, Mike.
