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charset="utf-8" Content-Transfer-Encoding: 8bit Content-Disposition: inline On Wed, May 28, 2025 at 10:49:36AM +0800, Zhaoyang Huang wrote: > On Wed, May 28, 2025 at 9:25 AM Hyesoo Yu wrote: > > > > On Mon, May 26, 2025 at 07:49:57PM +0800, Zhaoyang Huang wrote: > > > > Hello, Zhaoyang. > > > > I don't believe commit 1aaf8c was just intended to prevent an infinite loop. > > The commit was introduced to allow pinning CMA memory in the pKVM on AOSP. > > > > That leads me to question whether the assumption that CMA can be long-term pinned is actually valid. > That depends on the user of CMA, yes for my scenario since it worked > for the guest os. For common scenario such as the file/anon mapping, > the page will be judged as unpinnable for long-term and be migrated > out of CMA area. Your scenario and the common scenarios can not be distinguished from the kernel API's perspective. Even in common cases, the page may be in a non-LRU state temporiarily, and in such situations, pinning CMA can lead to bugs - we've encountered multiple issues because of this. > > > > In my opinion, it might be more appropriate to revert that commit 1aaf8c and instead ensure > > that pKVM avoids using CMA for memory that requires long-term pinning through GUP ? > It is not a pkvm issue but a defect of applying FOLL_LONGTERM over > non-LRU CMA pages. In include/linux/mm_types.h, the CMA should be migrated when FOLL_LONGTERM. * In the CMA case: long term pins in a CMA region would unnecessarily fragment * that region. And so, CMA attempts to migrate the page before pinning, when * FOLL_LONGTERM is specified. Given this, would it make sense to avoid using FOLL_LONGTERM in this code path ? > > > > Alternatively, instead of changing the current logic that prevents longterm GUP from pinning CMA, > > it would be better to propose a new patch that specifically addresses the pKVM scenario like adding new FOLL_flags ? > I don't think so. pin_user_pages is an exported API which can't make > assumptions over the caller. My point is not to base the patch on assumptions about the caller, but to define a clear mechanism that ensures safe behavior in the intended scenario. For example, you can add FOLL_NO_MIGRATION and skip to migrate unpinnable pages. Thanks, Regards. > > > > Thanks, > > Regards. > > > ------NB9Xbt2lYBAYtUThr5CB-M39J.YZyDXic4cnzgIuRipj9hAC=_d3265_ Content-Type: text/plain; charset="utf-8" ------NB9Xbt2lYBAYtUThr5CB-M39J.YZyDXic4cnzgIuRipj9hAC=_d3265_--