On Wed, Mar 25, 2020 at 7:09 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Wed, Mar 25, 2020 at 07:03:32PM +0100, Alexander Potapenko wrote: > > > I would suggest using bits in the section labelled: > > > > > > /* Unserialized, strictly 'current' */ > > > > The main problem is that |current| is unavailable in the interrupt > > context, so we'll also need to: > > - disable interrupts when preparing for a KMSAN internal memory > > allocation - sounds costly, huh? > > - store the context flag in a per-cpu variable in the case |current| > > is unavailable. > > It's not /unavailable/ ... it's whatever task happens to be running at > the time the interrupt is triggered. You can borrow its task_struct. That didn't come to my mind, interesting! > You'll have to save off the current value of the flag before setting it, > just like memalloc_nofs_save() does. > But this does rather call into question whether Michal's advice to use > task_struct is good advice to begin with. For memalloc_nofs/noio, > it works well this way because allocations in interrupt context are > inherently at a more restrictive context than task level. It's not clear > to me what this kmsan GFP flag is being used for, and whether allocations > that happen in interrupt context should inherit the kmsan setting. The idea behind this flag is as follows. KMSAN must allocate metadata pages for every allocation performed by alloc_pages(). Metadata allocations are also done via alloc_pages(), and for those no further metadata needs to be allocated. Thus the GFP flag is used to prevent the recursion in alloc_pages(). It is theoretically possible that a less restrictive allocation (without __GFP_NO_KMSAN_SHADOW) happens in an interrupt on top of a task performing a more restrictive allocation (with __GFP_NO_KMSAN_SHADOW). > I will have to read these patches more carefully to determine that; > I was really just responding to the "where can I find some free bits" > part of the question. Thanks for clarification. -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
