Re: [PATCH v3 01/14] virhostmem: Introduce virHostMemGetTHPSize()

On Fri, Apr 23, 2021 at 15:24:23 +0200, Michal Privoznik wrote: > New virHostMemGetTHPSize() is introduced which allows caller to > obtain THP PMD (Page Middle Directory) size, which is equal to > the minimal size that THP can use, taken from kernel doc > (Documentation/admin-guide/mm/transhuge.rst): > > Some userspace (such as a test program, or an optimized memory allocation > library) may want to know the size (in bytes) of a transparent hugepage:: > > cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size > > Since this size depends on the host architecture and the kernel > it won't change whilst libvirtd is running. Therefore, we can use > virOnce() and cache the value. Of course, we can be running under > kernel that has THP disabled or has no notion of THP at all. In > that case a negative value is returned to signal error. > > Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; > --- > src/libvirt_private.syms | 1 + > src/util/virhostmem.c | 63 ++++++++++++++++++++++++++++++++++++++++ > src/util/virhostmem.h | 3 ++ > tests/domaincapsmock.c | 9 ++++++ > 4 files changed, 76 insertions(+) [...] > diff --git a/src/util/virhostmem.c b/src/util/virhostmem.c > index ae42978ed2..89b31af3ca 100644 > --- a/src/util/virhostmem.c > +++ b/src/util/virhostmem.c > @@ -45,11 +45,14 @@ > #include "virstring.h" > #include "virnuma.h" > #include "virlog.h" > +#include "virthread.h" > > #define VIR_FROM_THIS VIR_FROM_NONE > > VIR_LOG_INIT("util.hostmem"); > > +static unsigned long long virHostTHPPMDSize; /* in kibibytes */ > +static virOnceControl virHostMemGetTHPSizeOnce = VIR_ONCE_CONTROL_INITIALIZER; > > #ifdef __FreeBSD__ > # define BSD_MEMORY_STATS_ALL 4 > @@ -920,3 +923,63 @@ virHostMemAllocPages(unsigned int npages, > > return ncounts; > } > + > +#if defined(__linux__) > +# define HPAGE_PMD_SIZE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size" > +static int Nothing checks the return value. > +virHostMemGetTHPSizeSysfs(unsigned long long *size) > +{ > + g_autofree char *buf = NULL; > + > + /* 1KiB limit is more than enough. */ > + if (virFileReadAll(HPAGE_PMD_SIZE_PATH, 1024, &buf) < 0) > + return -1; > + > + virStringTrimOptionalNewline(buf); > + if (virStrToLong_ull(buf, NULL, 10, size) < 0) { > + virReportError(VIR_ERR_INTERNAL_ERROR, > + _("unable to parse THP PMD size: %s"), buf); > + return -1; > + } > + > + /* Size is now in bytes. Convert to KiB. */ > + *size >>= 10; > + return 0; > +} > +#endif /* defined(__linux__) */ > + > + > +static void > +virHostMemGetTHPSizeOnceInit(void) > +{ > +#if defined(__linux__) > + virHostMemGetTHPSizeSysfs(&virHostTHPPMDSize); > +#else /* !defined(__linux__) */ > + VIR_WARN("Getting THP size not ported yet"); > +#endif /* !defined(__linux__) */ > +} > + > + > +/** > + * virHostMemGetTHPSize: > + * @size: returned size of THP in kibibytes > + * > + * Obtain Transparent Huge Page size in kibibytes. The size > + * depends on host architecture and kernel. Because of virOnce(), > + * do not rely on errno in case of failure. > + * > + * Returns: 0 on success, > + * -1 on failure. > + */ > +int > +virHostMemGetTHPSize(unsigned long long *size) > +{ > + if (virOnce(&virHostMemGetTHPSizeOnce, virHostMemGetTHPSizeOnceInit) < 0) This directly returns the return value from 'pthread_once' whose manual entry states: RETURN VALUE Upon successful completion, pthread_once() shall return zero; otherwise, an error number shall be returned to indicate the error. Which reads as if 'errno' is returned by the function which would make the '< 0' check wrong.